move old fnT+fnF versions of t_map, t_fold, etc. to a submodule

Rename as little as possible and keep the API.
Make all the necessary checks in Term and Decl.
Remove the duplicate code in Term but keep it elsewhere.
We will factorize the code as we go, without rush.

I love static typing!

My birthday gift for François.

in case of syntax error on reload, do not exit abruptly but displays a message and scroll to loc of the error