`sol!` tracking issue

[DaniPopes]

This is a tracking issue for missing sol! proc macro (and corresponding sol-types, syn-solidity) features.

Blockers

• [x] #68
• [x] #38
• [x] #55
• [x] #73

Nice-to-haves

• [x] #36
• [x] #37
• [x] #39
• [x] #82
• [x] #163
• [x] #157, #184
• [ ] #254
• [ ] #261