Slow build times: split up package, more optin

[wilbowma]

Several users are complaining about slow build times, causing them to avoid mttex when they can, occasionally causing them to copy/paste out of mttex to get specific features without slow build times.

This is a critical bug. Need to split up package and make more feature opt-in.

[wilbowma]

Slow build times can be somewhat alieviated by running "pdflatex" instead of "latexmk"

[wilbowma]

Plan: Split mttex into:

1. [ ] mttex/langs, which contains the \newlanguage and associated commands
2. [ ] mttex/smallify which provides interposition points for smallification, and a smallification flag. should look at packages like savetrees, too
3. [ ] mttex/smartlabels, which defines the fullref magic; need to look at hyperref, and cleverref et al see if this isn't duplicate code
4. [ ] Remove old sigplan/acm stuff that is handled by acmart now. ~mttex/stack, which defines the stack environments~ This is a tiny wrapper around align...~

To be removed:

1. [x] Theorem environments. This doesn't actually add much, and is defined by various documentclasses
2. [x] Font shorthands. Shorter code isn't better.

Not sure what to do with automagic title casing, todos and comments, etc.