feat(emacs/lean-mode): bind "C-c C-k" to quail-show-key

 - Use this to see how to type a unicode character
fix(emacs/lean-syntax): change comment symbols to be considered as punctuations

Please read the documentation of "modify-syntax-entry":

"The first character of NEWENTRY should be one of the following:
  _           symbol constituent
  .           punctuation
"

Close: #306
feat(frontends/lean): add [whnf] modifier to eval command
refactor(library/logic/cast): move heq declaration to a separate module

heq is be needed for some automatically generated constructions.
So, we want it available with the least number of dependencies.
refactor(library/data/prod): break into pieces to reduce dependencies

prod is needed for some automatically generated constructions.
So, it is important it is loaded in the environment as early as possible.
checkpoint