Add nulls, and more newdecl and type system features.

New null_t type has a constant "null", which can be assigned to objects.
Methodmaps can now be "nullable", accepting null_t types.
The delete keyword now zaps non-const lvalue inputs.
Fixed some newdecl arguments not accepting references.
Allow functag matching to coerce arguments and return values.
Allow functags to accept new-style types as a return value.

sourcepawn/compiler/sc2.c

@@ -1956,7 +1956,7 @@ char *sc_tokens[] = {
          "goto",
          "if", "int",
          "methodmap",
-         "native", "new",
+         "native", "new", "null", "__nullable__",
          "object", "operator",
          "public",
          "return",