Michelson: Type variable renaming

Context

This Merge-request is a proposal to rename stack variables with upper-case. The purpose is to makes it easier to read Michelson's GADT types parameters.

The purpose is purely documentation, it must not change the code semantics in anyway.

The biggest problem of this MR is the high potential of conflicts with any code touching michelson

Manually testing the MR

The MR is splitted in two commits, one changing the .ml file, the other changing the .mli file. So you can check, by applying only one of the two commits, that:

• old.ml + new .mli: the new interface does not relax type constraints that where in the former one (if you trust the former implementation to be as constraint as its interface).
• new.ml + old.mli: the new implementation does not create type constraints that where not in the former one.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR