Proto/Michelson: internal renaming of "union" into "or"

Context

The Michelson type "or" is referred to as "union" in the protocol to avoid clashing with OCaml's keyword "or".

This is inconsistent with the way all other Michelson types are treated.

This MR renames all occurrences of "union" into "or" except the few ones that would clash with the OCaml keyword which are renamed into "or_" instead.

Manually testing the MR

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