Add typestates package

[elijahr]

Adds typestates - compile-time state machine validation for Nim.

Checklist

• URL works and .nimble file present
• Package installable via nimble install https://github.com/elijahr/nim-typestates
• Builds with Nim 2.2.6 (latest stable)
• MIT licensed, no 3rd party dependencies
• Not a tiny library (11 modules, ~1800 LOC)
• Unique, specific name
• Contact email in nimble file
• Follows SemVer 2

Links

• Repository: https://github.com/elijahr/nim-typestates
• Documentation: https://elijahr.github.io/nim-typestates