fix: remove use of graphlib.TypeNode in type context

[seandstewart]

This simplifies the work to resolve routines for fields of structured types and reduces the overall footprint of the type context.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.97%. Comparing base (6bd7023) to head (e4742c0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/typelib/unmarshals/routines.py	57.14%	2 Missing and 1 partial ⚠️
src/typelib/ctx.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   96.92%   96.97%   +0.05%     
==========================================
  Files          21       21              
  Lines        1494     1486       -8     
  Branches      267      264       -3     
==========================================
- Hits         1448     1441       -7     
  Misses         27       27              
+ Partials       19       18       -1     

Flag	Coverage Δ
unittests	96.70% <90.00%> (+0.04%)	⬆️

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