Kill cstdstring

Builds on night199uk's work to kill off functions inside CStdString.

The main one left after this PR goes in is .Equals() (of which there are rather a number of instances).

It would be useful to have review of this.  I've reviewed all except Format() (which I'll do over the coming days), but another pair of eyes would be useful.  It's not fun work (I strongly suggest reviewing a single commit at a time).

There's lots of reasons why even when this is in CStdString will stay around for quite a long time, but this at least gets rid of the bulk of the convenience, which should mean new code will err to std::string instead.