Implement Str.parse-names

Functionality:
- similar to "\c[BELL, BLACK HEART SUIT]", except can be used by
    users at runtime, without requiring EVAL/nqp in user's code
- Unlike \c[…] does not allow comments or anything similarly isoteric.
    The idea is this will process some sort of user input, so we want
    it just as liberal as necesary (whitespace around char names is allowed).

Naming:
- the name follows Str.parse-base that parses a string containing numerals
    in some base; whereas this routine parses a string of character names

src/core/Exception.pm

@@ -1810,6 +1810,14 @@ my class X::Phaser::PrePost is Exception {
     }
 }
 
+my class X::Str::InvalidCharName is Exception {
+    has $.name;
+    method message() {
+        $!name.chars ?? "Unrecognized character name [{$!name}]"
+                     !! "Cannot use empty name as character name"
+    }
+}
+
 my class X::Str::Numeric is Exception {
     has $.source;
     has $.pos;

src/core/Str.pm

@@ -1,6 +1,7 @@
 my class Cursor {... }
 my class Range  {... }
 my class Match  {... }
+my class X::Str::InvalidCharName { ... }
 my class X::Str::Numeric  { ... }
 my class X::Str::Match::x { ... }
 my class X::Str::Subst::Adverb { ... }
@@ -1353,6 +1354,24 @@ my class Str does Stringy { # declared in BOOTSTRAP
         }
     }
 
+    method parse-names(Str:D:) {
+        my     \names := nqp::split(',', self);
+        my int $elems  = nqp::elems(names);
+        my int $i      = -1;
+        my str $res    = '';
+        nqp::while(
+            nqp::islt_i( ($i = nqp::add_i($i,1)), $elems ),
+            ($res = nqp::concat($res,
+                nqp::unless(
+                    nqp::getstrfromname(nqp::atpos(names, $i).trim),
+                    X::Str::InvalidCharName.new(
+                        :name(nqp::atpos(names, $i).trim)
+                    ).fail
+            ))),
+        );
+        $res
+    }
+
     multi method split(Str:D: Regex:D $pat, $limit is copy = Inf;;
       :$v is copy, :$k, :$kv, :$p, :$skip-empty) {
 
@@ -2972,6 +2991,8 @@ sub chrs(*@c) returns Str:D {
 proto sub parse-base(|) { * }
 multi sub parse-base(Str:D $str, Int:D $radix) { $str.parse-base($radix) }
 
+sub parse-names(Str:D $str) { $str.parse-names }
+
 proto sub substr(|) { * }
 multi sub substr(Str:D \what, Int:D \start) {
     my str $str  = nqp::unbox_s(what);

t/spectest.data

@@ -1089,6 +1089,7 @@ S32-str/numeric.t
 S32-str/ords.t
 S32-str/pack.t
 S32-str/parse-base.t
+S32-str/parse-names.t
 S32-str/pos.t
 S32-str/rindex.t
 S32-str/samemark.t