Save space saving sizes of dynamic fields only

[twiggler]

Currently, actual sizes of all fields are stored inside the _sizes private member. This can cause high memory usage in some applications.

This PR only stores the sizes of dynamic fields. Since most fields are static, we significantly reduce memory usage, utilities like dumpstruct still work as intended.

Alternative solution considered: Remove _sizes completely. However, no alternative way has been found to reconstruct the number of bytes read. This would result in loss of the pretty colors in dump_struct, for example

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.88525% with 8 lines in your changes missing coverage. Please review.

Project coverage is 92.17%. Comparing base (e990b53) to head (dc39c7b).

Files with missing lines	Patch %	Lines
dissect/cstruct/types/structure.py	88.63%	5 Missing :warning:
dissect/cstruct/parser.py	57.14%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   92.24%   92.17%   -0.07%     
==========================================
  Files          21       21              
  Lines        2477     2506      +29     
==========================================
+ Hits         2285     2310      +25     
- Misses        192      196       +4     

Flag	Coverage Δ
unittests	92.17% <86.88%> (-0.07%)	:arrow_down:

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[JSCU-CNI]

Could this change be documented, e.g. using a CHANGELOG file entry?