Implement .fc method and fc(...) sub.

Performs the Unicode foldcase operation. Unlike uc/tc/lc, it already
understands full mappings (where one grapheme may expand to many). It
is made available on JVM too, but is incomplete there.

src/core/Cool.pm

@@ -100,6 +100,10 @@ my class Cool { # declared in BOOTSTRAP
         self.Str.tc
     }
 
+    method fc() {
+        self.Str.fc
+    }
+
     method tclc() {
         self.Str.tclc
     }
@@ -347,6 +351,7 @@ sub lc(Cool $s)                    { $s.lc }
 sub ord(Cool $s)                   { $s.ord }
 sub uc(Cool $s)                    { $s.uc }
 sub tc(Cool $s)                    { $s.tc }
+sub fc(Cool $s)                    { $s.fc }
 sub tclc(Cool $s)                  { $s.tclc }
 
 sub indices(Cool $s,$needle,$pos=0,:$overlap) {

src/core/Str.pm

@@ -1435,6 +1435,14 @@ my class Str does Stringy { # declared in BOOTSTRAP
         self.Str
     }
 
+    proto method fc(|) { * }
+    multi method fc(Str:D:) {
+        nqp::p6box_s(nqp::fc($!value));
+    }
+    multi method fc(Str:U:) {
+        self.Str;
+    }
+
     proto method tclc(|) { * }
     multi method tclc(Str:D:) {
         nqp::p6box_s(nqp::tclc($!value))