Make Structure.fields a dictionary

[Schamper]

This PR makes Structure.fields a dictionary, rather than a list. A dictionary mapping of fields is generally more useful to end-users, so this change makes that more easily available.

The list of fields is still available at __fields__ and is still considered the "single source of truth" of the structure contents. Because this change effectively makes fields behave exactly like the old lookup, I've removed __lookup__ and moved that into lookup.

This PR merges into the union editting v4 PR because it relies on some changes in there.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (324a364) 90.87% compared to head (597cffa) 90.90%.

Files	Patch %	Lines
dissect/cstruct/cstruct.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                  @@
##           v4-union-editing      #52      +/-   ##
====================================================
+ Coverage             90.87%   90.90%   +0.03%     
====================================================
  Files                    19       19              
  Lines                  2027     2034       +7     
====================================================
+ Hits                   1842     1849       +7     
  Misses                  185      185              

Flag	Coverage Δ
unittests	90.90% <96.55%> (+0.03%)	⬆️

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