.samemark: Support emoji, flags and Prepend

.samemark` is currently broken. it assumes the first codepoint of a grapheme is the "base", and *anything* occuring afterward is a "mark".
(below the `*` is used in the same meaning as regex, and things in brackets are single codepoint tokens)

**How samemark assumes the world exists**
`[base] [mark]*`

**The real world it is actually more like**
`[mark]*[base][mark]*`

**But** then you have emoji, flags etc.
`[Regional Indicator][Regional Indicator]`
**or**
`[Base Emoji]([ZWJ][Emoji])*`

What I think is the most reasonable solution is to treat `Grapheme_Cluster_Break=Extend` and `Grapheme_Cluster_Break=Prepend` as “mark”’s. This would cause it to treat flags or emoji sequences as the "base", since it's not really separable the same way accent marks are separable.

This does not address the fact that "\c[Canada]".samemark("é") would give you a Canadian flag with an accent mark on it, and why anybody would desire this. As this is how samemark has always worked it doesn't make sense to change this.

Fixes Raku/problem-solving#61

src/core.c/Str.pm6

@@ -2054,6 +2054,44 @@ my class Str does Stringy { # declared in BOOTSTRAP
     }
 
 #?if !jvm
+    sub get-parts ($nfd is raw) {
+      my int $p = 0;
+      my int $last-prepend = -1;
+      my int $first-extend = -1;
+      while $p < nqp::elems($nfd) &&
+        nqp::iseq_s('Prepend', nqp::getuniprop_str(
+          nqp::atpos_i($nfd, $p),
+          nqp::unipropcode('gcb'))) {
+        $last-prepend = $p;
+        $p = nqp::add_i($p, 1)
+      }
+      $p = nqp::elems($nfd) - 1;
+      while 0 <= $p && nqp::atpos_i($nfd, $p).uniprop('gcb') eq 'Extend' {
+        $first-extend = $p;
+        $p = nqp::sub_i($p, 1)
+      }
+      my $extends :=
+        nqp::if(($first-extend == -1),
+          nqp::create(NFD),
+          nqp::slice($nfd, $first-extend, nqp::sub_i(nqp::elems($nfd), 1))
+        );
+      my $prepends :=
+        nqp::if(($last-prepend == -1),
+          nqp::create(NFD),
+          nqp::slice($nfd, 0, $last-prepend)
+        );
+      my $base :=
+        nqp::slice(
+          $nfd,
+          nqp::if(($last-prepend == -1),
+            0,
+            nqp::add_i($last-prepend, 1)),
+          nqp::if(nqp::iseq_i($first-extend, -1),
+            nqp::sub_i(nqp::elems($nfd), 1),
+            nqp::sub_i($first-extend, 1))
+      );
+      return nqp::list($prepends, $base, $extends);
+    }
     method samemark(Str:D: Str:D $pattern --> Str:D) {
         nqp::if(
           nqp::chars(nqp::unbox_s($pattern)),        # something to work with
@@ -2069,25 +2107,29 @@ my class Str does Stringy { # declared in BOOTSTRAP
               nqp::islt_i(($i = nqp::add_i($i,1)),$marks-elems),
               nqp::bindpos_s($result,$i,              # store the result of:
                 nqp::stmts(
-                  (my $marks-nfd := nqp::strtocodes(  # char + accents of mark
+                  (my $pattern-nfd := nqp::strtocodes(  # char + accents of mark
                     nqp::atpos($marks,$i),
                     nqp::const::NORMALIZE_NFD,
                     nqp::create(NFD)
                   )),
-                  nqp::shift_i($marks-nfd),           # lose the char
-                  (my $marks-base := nqp::strtocodes( # char + accents of base
+                  # Make NFD array of input's
+                  (my $input-nfd := nqp::strtocodes( # char + accents of base
                     nqp::atpos($base,$i),
                     nqp::const::NORMALIZE_NFD,
                     nqp::create(NFD)
                   )),
-                  nqp::strfromcodes(                  # join base+rest of marks
-                    nqp::splice(
-                      $marks-base,
-                      $marks-nfd,
-                      1,
-                      nqp::sub_i(nqp::elems($marks-base),1)
-                    )
-                  )
+                  (
+                  my $input-parts := get-parts($input-nfd);
+                  my $pattern-parts := get-parts($pattern-nfd);
+                  # This is used below as well
+                  my $pattern-prepends := nqp::atpos($pattern-parts, 0);
+                  my $pattern-extends := nqp::atpos($pattern-parts, 2);
+                  my $out := nqp::create(NFD);
+                  nqp::splice($out, $pattern-prepends, 0, nqp::elems($pattern-prepends));
+                  nqp::splice($out, nqp::atpos($input-parts, 1), nqp::elems($out), 0);
+                  nqp::splice($out, $pattern-extends, nqp::elems($out), 0);
+                  ),
+                  nqp::strfromcodes($out),
                 )
               )
             ),
@@ -2097,17 +2139,28 @@ my class Str does Stringy { # declared in BOOTSTRAP
               nqp::islt_i(($i = nqp::add_i($i,1)),$base-elems),
               nqp::bindpos_s($result,$i,              # store the result of:
                 nqp::stmts(
-                  ($marks-base := nqp::strtocodes(    # char+all accents of base
+                  ($input-nfd := nqp::strtocodes(    # char+all accents of base
                     nqp::atpos($base,$i),
                     nqp::const::NORMALIZE_NFD,
                     nqp::create(NFD)
                   )),
+                  (
+                    my $input-base := nqp::atpos(get-parts($input-nfd), 1);
+                  ),
                   nqp::strfromcodes(                  # join base+rest of marks
-                    nqp::splice(
-                      $marks-base,
-                      $marks-nfd, # NOTE: state of last iteration previous loop
-                      1,
-                      nqp::sub_i(nqp::elems($marks-base),1)
+                    nqp::stmts(
+                      nqp::splice(
+                        $input-base,
+                        $pattern-extends, # NOTE: state of last iteration previous loop
+                        nqp::elems($input-base),
+                        0
+                      ),
+                      nqp::splice(
+                        $input-base,
+                        $pattern-prepends, # NOTE: state of last iteration previous loop
+                        0,
+                        0
+                      )
                     )
                   )
                 )