Replace '.' characters by '_' in enum type names since OCaml doesn't allow to use '.' in any kind of identifiers.

Created by: cgensoul

This PR replaces '.' characters by '_' ones in generated type names. More precisely this only affects types generated in the support/Enums.ml file.

It brings us one step closer to being able to generated compilable code for https://raw.githubusercontent.com/stripe/openapi/master/openapi/spec3.yaml

Fixes #7757 (closed).