refactor(core): replace attribute-related panics with warnings

[imrn99]

• sew methods now behave like link methods even if there are no attribute defined (instead of panicking)
• update UnknownAttributeStorage implementations to print warnings instead of panick when attributes are missing
• replace AttributeUpdate::merge_undefined with two distinct functions depending if there's one or no attribute available

The PR has mixed content, the changes I started required more and more stuff to be changed as a consequence

Scope

• [x] Code

Type of change

• [x] Refactor

Other

• [x] Breaking change

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.41758% with 16 lines in your changes missing coverage. Please review.

Project coverage is 81.97%. Comparing base (4a5ed63) to head (95a0aea).

Files with missing lines	Patch %	Lines
honeycomb-core/src/attributes/collections.rs	71.42%	14 Missing :warning:
honeycomb-kernels/src/grisubal/model.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #178 +/- ## ======================================= Coverage 81.96% 81.97% ======================================= Files 32 32 Lines 4503 4538 +35 ======================================= + Hits 3691 3720 +29 - Misses 812 818 +6 ```

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