Proto-env: remove unused modules from stdlib-compat

Context

We can get rid of most stdlib_compat modules with very little effort. This MR does that by introducing a v2/seq.mli signature and simply removing references to Stdlib. in signatures.
NOTE: in this specific case it is ok to change the existing environment because the environment cannot be activated again and the changes only introduce types (and modules that only include types) which rules out changes in semantic.

Also rename V_all into Tezos_protocol_environment_sigs_internal (which is a more descriptive name for the lib).

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