feat(collections/unstable): accept Iterable in interleave

[tomas-zijdemans]

interleave now widens the API to Iterable without extra cost for array callers. This is in line with the general direction of the collections module. interleave is very close to zip that does not support Iterable, but looking at the module as a whole, I think it's zip that needs to change.

Also added a two-array fast path (no inner loop, no per-pair bounds check) and a branch for equal-length inputs where the length guard is unnecessary. This should help with two very common use cases. The general unequal-length path is unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (1cd63ca) to head (b9a16ff).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7088   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50490    50518   +28     
  Branches     8949     8959   +10     
=======================================
+ Hits        47669    47698   +29     
  Misses       2249     2249           
+ Partials      572      571    -1     

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