[RFC] Refactor MCSTR & MCNAME to use MCAutoStringRef & MCNewAutoNameRef.

This has two advantages:

1. MCStringRefs and MCNameRefs created using the MCSTR and MCNAME
   macros are automatically cleaned up when they're no longer needed.

2. There is a more convenient syntax for creating an MCAutoStringRef
   from a C literal string:

       MCAutoStringRef t_literal("My literal string here");