Fix "Missing serialize REPR function for REPR NativeRef (StrLexRef)"

ASN::META parses a definition with a grammar and creates classes and methods
from the parsed description. This leads to a StrLexRef getting used as a method
name which ends up in the serialization context. Decont the names of added
methods to ensure we have a plain old string.

Fixes GH #3045

src/Perl6/Metamodel/MethodContainer.nqp

@@ -17,6 +17,7 @@ role Perl6::Metamodel::MethodContainer {
     method add_method($obj, $name, $code_obj) {
         # Ensure we haven't already got it.
         $code_obj := nqp::decont($code_obj);
+        $name := nqp::decont_s($name);
         if nqp::existskey(%!methods, $name) || nqp::existskey(%!submethods, $name) {
             nqp::die("Package '"
               ~ self.name($obj)

src/Perl6/Metamodel/PrivateMethodContainer.nqp

@@ -5,6 +5,7 @@ role Perl6::Metamodel::PrivateMethodContainer {
 
     # Adds a private method.
     method add_private_method($obj, $name, $code) {
+        $name := nqp::decont_s($name);
         if nqp::existskey(%!private_methods, $name) {
             nqp::die("Private method '$name' already declared in package " ~
                 self.name($obj));