Add nulls, and more newdecl and type system features.

New null_t type has a constant "null", which can be assigned to objects.
Methodmaps can now be "nullable", accepting null_t types.
The delete keyword now zaps non-const lvalue inputs.
Fixed some newdecl arguments not accepting references.
Allow functag matching to coerce arguments and return values.
Allow functags to accept new-style types as a return value.
Merge branch 'master' into nullable
Fix AMBuildScript compatibility issue with older objdirs.
Rename the null tag to null_t.
Mark the expression before flushing the staging buffer.
Merge pull request #73 from alliedmodders/nullable

Add nullable types and tighten up some new type system semantics.