Need a more powerful and clean signature merging functionality

[thorwhalen]

Sig.merge_with_sig is horrendously designed and not even that expressive.

We need to think more carefully of its design and interaction with any signature-merging functionalities (for example in meshed).

There are some other signature merging functions laying around. Notably, meshed.util.conservative_parameter_merge which is fairly clean, but only takes care of kind, default and annotation conflicts.

[thorwhalen]

Possible duplicate issue: Write a general parameters merging function.