[m-dev.] for review: bug fixes & cleanups in runtime/*

[Zoltan Somogyi]

> Unfortunately most of these macros were still used in the code.
> For the most part, I have modified the code to avoid using the old names.
> However, I left push() and pop() in there since they are
> used in mercury_wrapper.c.
> 
> Here's a diff that addresses those concerns.

That diff is fine as far as it goes. However, the compiler still generates
references to the bt_* macros.

Zoltan.