Add CSS calc

[kokoISnoTarget]

Objective

Fixes: #225

Context

https://github.com/DioxusLabs/taffy/pull/232#issuecomment-1501163557 https://drafts.csswg.org/css-values/#funcdef-calc

Feedback wanted

Todo

• [ ] Change RELEASES.md
• [ ] Testing
  • [x] Finish Parser for tests
  • [ ] Add tests
  • [ ] Test on other systems because of fun pointer stuff:
  • [x] 32bit systems
  • [ ] Little and Big-endian systems
  • [x] Non x86 platforms (Arm and whatever)
  • [ ] Windows and macOS
  • [x] Miri
  • [x] Look into Strict Provenance / sptr
• [x] Add basic remaining CSS math functions
• [x] Documentation (not the best)
• [ ] Performance
  • [ ] Don't be 15% slower
• [x] Clean things up
• [ ] Impl Serde

[nicoburns]

@kokoISnoTarget I've been reading some of your commits, and regarding the "union", I suspect you may actually want a newtype around a u32 (or u64 if you need 64 bits). You can then use type casts and bit manipulation operators to access the tag and the value(s).

[kokoISnoTarget]

@nicoburns I'm not quite sure what the problem of my current approach is, but I appreciate the feedback.