Implement named parameters in signatures (PPC0024)

This adds a major new ability to subroutine signatures, allowing callers
to pass parameters by name/value pairs rather than by position.

  sub f ($x, $y, :$alpha, :$beta = undef) { ... }

  f( 123, 456, alpha => 789 );

Originally specified in

  https://github.com/Perl/PPCs/blob/main/ppcs/ppc0024-signature-named-parameters.md

This feature is currently considered experimental.

Author: leonerd

dump.c

@@ -1804,6 +1804,11 @@ S_do_op_dump_bar(pTHX_ I32 level, UV bar, PerlIO *file, const OP *o,
                 S_opdump_indent(aTHX_ o, level, bar, file, "  PARAM [%zd] ANON\n",
                         i);
         }
+        for(size_t i = 0; i < aux->n_named; i++) {
+            struct op_multiparam_named_aux *named = aux->named + i;
+            S_opdump_indent(aTHX_ o, level, bar, file, "  NAMEDPARAM <%.*s> PADIX = %" UVuf "%s\n",
+                    named->namelen, named->namepv, named->padix, named->is_required ? "" : " OPT");
+        }
 
         if(aux->slurpy)
             S_opdump_indent(aTHX_ o, level, bar, file, "SLURPY = '%c' PADIX = %" UVuf "\n",

embed.fnc

@@ -3209,6 +3209,11 @@ p	|void	|sub_crush_depth|NN CV *cv
 : Used in perly.y
 p	|void	|subsignature_append_fence_op				\
 				|NN OP *o
+p	|void	|subsignature_append_named				\
+				|NN const char *paramname		\
+				|PADOFFSET padix			\
+				|OPCODE defmode 			\
+				|NULLOK OP *defexpr
 p	|void	|subsignature_append_positional 			\
 				|PADOFFSET padix			\
 				|OPCODE defmode 			\

embed.h

@@ -1163,6 +1163,7 @@
 #   define sighandler3                          Perl_sighandler3
 #   define sub_crush_depth(a)                   Perl_sub_crush_depth(aTHX_ a)
 #   define subsignature_append_fence_op(a)      Perl_subsignature_append_fence_op(aTHX_ a)
+#   define subsignature_append_named(a,b,c,d)   Perl_subsignature_append_named(aTHX_ a,b,c,d)
 #   define subsignature_append_positional(a,b,c) Perl_subsignature_append_positional(aTHX_ a,b,c)
 #   define subsignature_append_slurpy(a,b)      Perl_subsignature_append_slurpy(aTHX_ a,b)
 #   define subsignature_finish()                Perl_subsignature_finish(aTHX)