Strengthen restrictions on module names

https://github.com/FStarLang/FStar/issues/587#issuecomment-233537253 suggested to use `_` in module names.
However this potentially leads to conflicts with the OCaml extraction since the dot `.` turns into `_` in the OCaml module names.
If I have 2 modules "Module.Module.fst" and "Module_Module.fst" that are extracted together, one will overwrite the other.
Should the underscore be banned ?
Another solution would be to allow underscores before numbers and small caps only, but it may be a lot of trouble for not much.

Start by examining the module-name validation and OCaml extraction behavior described in this issue, focusing on how dots and underscores are translated. Reproduce the collision between Module.Module.fst and Module_Module.fst, then determine whether the accepted naming rules prevent it and add coverage for the chosen restriction.

Written by the indexing model from the issue text.