compiler: Memoize GetType() on TExpr

This massively reduces the time to generate intermediate code on large Orly source files.

Thanks Joris (@jmlvanre) for finding / coming up with this (And initial patch). This changes the implementation pattern of that patch, but does the same thing.
Merge pull request #242 from cmaloney/intern-get-type

compiler: Memoize GetType() on TExpr