Move genType inside the compiler, and trade-offs.

[cristianoc]

Moving the project inside the compiler can enable deeper integration and solve a number of longstanding issues caused by treating compilation as a black box and genType as a post-processing. Also, it means staying in sync with the compiler version automatically.

This also means making it as lean as possible, and reduce tests to the essential (to run alongside compiler tests). To do that, the flow and uptyped backends will be removed, and anything not necessary to the integration as well.

To avoid disrupting existing users, this repository can stay unchanged, so it's still possible to apply fixes if necessary. The catch is, for people still using standalone genType it will not be possible to do a compiler upgrade in future, as new compiler versions will have genType built in.

[cristianoc]

• [x] remove flow and untype back-ends
• [x] remove Js.t support
• [x] importPath: only keep Relative
• [x] remove reasonReactPath
• [x] remove bsCurryPath config
• [x] remove propTypes
• [x] remove fileHeader config
• [ ] see what to do with exportInterfaces, either remove, or leave and make it better later