Added AST and Function classes.
Now, we parse the whole file and build up the AST for all of the
functions in the Module before we emit IR for the functions (vs. before,
when we generated IR along the way as we parsed the source file.)
- Renamed stdlib-sse.ll to builtins-sse.ll (etc.) in an attempt to better indicate
the fact that the stuff in those files has a role beyond implementing stuff for
the standard library.
- Moved declarations of the various __pseudo_* functions from being done with LLVM
API calls in builtins.cpp to just straight up declarations in LLVM assembly
language in builtins.m4. (Much less code to do it this way, and more clear what's
going on.)
