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
Drop support for everything but BOMless UTF-8
(Only) support reading directly from memory
Make the check for blank lines less horribly slow
Make NextInt even less slow
Don't remove \r and \n from dialogue bodies

Aegisub already handles this.