Make handling of names in mlang nominal

[elegios]

This PR replaces #809, to be able to separately merge the nominal changes without the updates because of the new type-checker.

In this PR:

• Nominal handling
• Merging of definitions using with, though this excludes renaming presently; it's only allowed if all things getting merged have the same original string name.
• Fixing the compiler and the test suite such that it compiles under this slightly more strict handling of things.

Not included, but previously discussed:

• Typechecker-related changes, these will be presented in a separate PR
• Renaming of declarations/definitions from extended language fragments (as mentioned above; merging things with different string names requires a rename along the way).

Important: The changes here should be checked against the other dependent repositories as well. We believe we found and fixed everything before the paper deadline, but those PRs haven't necessarily been merged yet:

• Various fixes for paper evaluation miking-dppl#148
• TreePPL doesn't seem to have a PR, so it might not need fixes, but this should be confirmed.
• Same for ProbTime

Finally, it's worth mentioning that the PRs in the dependent repos can be merged before this one; the required changes should be fully backwards-compatible.