Move Michelson representation modules above the Alpha_context abstraction barrier

Context

Currently owned by @sventimir.

Some modules defining Michelson representation types (for the Michelson types nat, int, timestamp, and string) are below the abstraction barrier for no good reason. This module moves them above the barrier to:

• make the signature of the Alpha_context module smaller, and
• prevent these module to accidentally break the structural invariants of the context.

Manually testing the MR

This is a refactoring MR with absolutely no expected impact on the feature set nor performance.

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