Improve `codegen` submodule (Resolves #36)

[P3ridot]

#36

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.66667% with 78 lines in your changes are missing coverage. Please review.

Project coverage is 70.84%. Comparing base (8a4d131) to head (267c9ba).

Files	Patch %	Lines
...a_lang/utilities/inject/CodegenFieldsInjector.java	0.00%	40 Missing :warning:
...g/panda_lang/utilities/inject/DefaultInjector.java	25.00%	15 Missing :warning:
.../org/panda_lang/utilities/inject/CodegenCache.java	77.36%	8 Missing and 4 partials :warning:
...g/utilities/inject/CodegenConstructorInjector.java	69.24%	2 Missing and 2 partials :warning:
...a_lang/utilities/inject/CodegenMethodInjector.java	69.24%	2 Missing and 2 partials :warning:
...g/utilities/inject/DefaultConstructorInjector.java	92.31%	1 Missing :warning:
...a_lang/utilities/inject/DefaultFieldsInjector.java	88.89%	1 Missing :warning:
...a_lang/utilities/inject/DefaultMethodInjector.java	85.72%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #38      +/-   ##
============================================
- Coverage     76.50%   70.84%   -5.66%     
- Complexity      141      151      +10     
============================================
  Files            26       29       +3     
  Lines           468      528      +60     
  Branches         52       55       +3     
============================================
+ Hits            358      374      +16     
- Misses           93      141      +48     
+ Partials         17       13       -4     

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