[tapl] Simple Bool

[at15]

From https://github.com/at15/mini-impl/issues/6

Before implement subtyping, let's first finish typing, this match to simplebool and Chapter 10 in TAPL

• [x] read OCaml code
• [x] syntax
• [x] parser
• [x] ast builder
• [x] type checker
• [ ] evaluator

[at15]

I think I have to start working on real Reika implementation now, there is little time left in this quarter .... put this issue in back log