Improve the bootstrap

The objective of the seires is to simplify the boostrap by using `nith` with some optimizations to do the job.

To achieve this objective:

* commit 1. Fix nith to produce a functional nitg by removing a usage of advanced FFI
* commis 2-4 : Fix option `--direct-call-monomorph` by avoiding lose of typing information in AST transformation
* commit 5: use a partial nitg before a final nitg ; `nith` -> `nitg_0` -> `nitg`
* commit 6: use nith in c_src