The rewrite her discards the functions that did direct, recursive
expansion of macro values. Instead, the parser now pushes the macro
definition string over to a stack of buffers for the lexer. This way,
macro expansion gets access to all parsing machinery.
This isn't a small change, but the result is simpler than before (I
think). It passes the entire test suite, including the four tests
added with the previous commit that were failing before.
The lexer was previously using strdup (expecting the parser to free),
but is now more consistent, easier to use, and slightly more efficent
by using talloc along with the parser.
Also, we add xtalloc and xtalloc_strdup wrappers around talloc and
talloc_strdup to put all of the out-of-memory-checking code in one
place.
