Replace symbol proxies with type symbols.

Proxies were rather hacky and complicated, and only existed as a
workaround for oddities with constructors. This patch replaces them with
actual type symbols, a very tiny step to semantically getting rid of tags.

This greatly simplifies how we implement constructors, and paves the way
for using methodmap symbols in field expressions.

Since non-value symbols are new to spcomp1, we place a rather
pigeonholed check into primary() to make sure non-value symbols don't
escape into expressions.
Add static method support to methodmaps.
Revert adminmenu API changes.
Merge pull request #219 from alliedmodders/static-methods

Add support for static methods on methodmaps.