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.

plugins/include/handles.inc

@@ -78,7 +78,7 @@ native Handle:CloneHandle(Handle:hndl, Handle:plugin=INVALID_HANDLE);
 /**
  * Helper for object-oriented syntax.
  */
-methodmap Handle
+methodmap Handle __nullable__
 {
 	public Clone() = CloneHandle;
 	public ~Handle() = CloseHandle;