Make Pair.raku about 15% faster

- formerly known as Pair.perl
- adds a Str.is-identifier method that returns True matches identifier syntax
  - since this appears to be generally useful, it seems like a good addition

src/core.c/Pair.pm6

@@ -105,10 +105,10 @@ key of the Pair should be a valid method name, not '$method'."
         })
     }
 
-    multi method raku(Pair:D: :$arglist) {
+    multi method raku(Pair:D: Bool:D :$arglist = False) {
         self.rakuseen('Pair', -> :$arglist {
             nqp::istype($!key, Str) && nqp::isconcrete($!key)
-              ?? !$arglist && $!key ~~ /^ [<alpha>\w*] +% <[\-']> $/
+              ?? nqp::not_i($arglist) && $!key.is-identifier
                 ?? nqp::istype($!value,Bool) && nqp::isconcrete($!value)
                    ?? ':' ~ '!' x !$!value ~ $!key
                    !! ':' ~ $!key ~ '(' ~ $!value.raku ~ ')'

src/core.c/Str.pm6

@@ -37,6 +37,17 @@ my class Str does Stringy { # declared in BOOTSTRAP
         nqp::bindattr_s(self, Str, '$!value', nqp::unbox_s($value))
     }
 
+    method is-identifier(Str:D: ) {
+        for self.split: ("-","'") -> str $part {
+            return False
+              unless nqp::chars($part) && nqp::findnotcclass(
+                nqp::const::CCLASS_ALPHABETIC,
+                $part,0,nqp::chars($part)
+              ) == nqp::chars($part)
+        }
+        True
+    }
+
     multi method Bool(Str:D: --> Bool:D) {
         nqp::hllbool(nqp::chars($!value));
     }