Rewrite CTextFile::ReadString

Drop ANSI support since it was used nowhere, purge stuff for non-Unicode
builds since it's been a very long time since there was any reason to do
such a thing, and make it not absurdly slow.

Reduces the time to load an arbitrary test file from 824ms to 180ms.
Make NextInt less slow

About a 5% speedup on loading an arbitrary test file.
Mark stuff not used outside of STS.cpp as static