Implement type-level string append function

[krame505]

This adds a TApp type pseudo-constructor like TAdd and friends, for appending type-level strings.

[quark17]

Looks good! I'm a tad concerned that TApp can be misread as type application (like BSC's TAp), but I'm not sure that I have a better suggestion.

[mieszko]

Looks good! I'm a tad concerned that TApp can be misread as type application (like BSC's TAp), but I'm not sure that I have a better suggestion.

Yeah TApp looks to me like application of some kind. Why not just TAppend? Unambiguous, and not too long.

[krame505]

I went with TApp mostly for consistency, the other type-level functions are all abbreviated to 3 letters: TAdd, TSub, TMul, TDiv, TLog, TExp, TMax, TMin.

TAppend is a little more verbose. No strong objection, though.