Convert lexer to talloc and add xtalloc wrappers.

Convert lexer to talloc and add xtalloc wrappers.

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.