refactor: move iterator into new module

[tlambert03]

This moves the iter_sequence function into a new module. Both for modularity, and also because I might play with speeding it up via mypyc

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (58c3343) 96.67% compared to head (af4c503) 96.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   96.67%   96.72%   +0.05%     
==========================================
  Files          12       13       +1     
  Lines         721      732      +11     
==========================================
+ Hits          697      708      +11     
  Misses         24       24              

Files Changed	Coverage Δ
src/useq/_iter_sequence.py	100.00% <100.00%> (ø)
src/useq/_mda_sequence.py	97.50% <100.00%> (-1.27%)	⬇️
src/useq/_utils.py	96.15% <100.00%> (+1.70%)	⬆️

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