Fix issue tracking recursive references to custom annotations

[bgirardeau]

Custom annotations (introduced in #110) allow users to define arbitrary annotations that can be used by calling an autogenerated processor for the Python backend. The Python backend code generation recursively searches for and includes custom annotation types for a backend, but the intermediate code generator does not track references recursively. This leads to missing imports in the Python code generation in some cases.

This change moves the recursive search into the intermediate code generator and puts the result in a new data type field for use in code generation by backends.

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Merging #197 into master will increase coverage by 0.55%.
The diff coverage is 96.36%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   51.17%   51.73%   +0.55%     
==========================================
  Files          37       37              
  Lines        8351     8367      +16     
  Branches     1781     1786       +5     
==========================================
+ Hits         4274     4329      +55     
+ Misses       3757     3724      -33     
+ Partials      320      314       -6     

Flag	Coverage Δ
#unit	51.73% <96.36%> (+0.55%)	⬆️

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Impacted Files	Coverage Δ
stone/backends/python_client.py	38.37% <ø> (ø)
stone/backends/python_types.py	35.97% <80.00%> (+0.33%)	⬆️
stone/frontend/ir_generator.py	80.57% <97.50%> (+1.05%)	⬆️
stone/ir/data_types.py	80.57% <100.00%> (+2.51%)	⬆️
stone/frontend/parser.py	92.04% <0.00%> (+0.61%)	⬆️
stone/ir/api.py	84.02% <0.00%> (+2.06%)	⬆️

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