fix: Perform choices validation after parse/mapping time.

[DanCardin]

Fixes #170.

Although i still want to look at this a little bit more. this does add some weird code duplication/different branches. Although I suppose some of that already existed before, it was just built into the parser.

In order to avoid needing to physically manipulate the annotations, this PR drops the nicer "choices" invalid choice: 'thename' (choose from 'one', 'two', 'three', 4)-style error messages for annotations like Union[Literal['one'], Literal['two']], because it becomes impractical to support.

Instead such annotations get the union-like error message

invalid value for 'name': possible variants
 - literal['one']: invalid choice: 'foo'
 - literal['two']: invalid choice: 'foo'

which is still clear, but not as nice and concise. I think given that unions of only literals (which was the condition for this to work can and probably should be trivially converted to just a literal, this seems like a worthwhile tradeoff.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (0c1b038) to head (8189a30).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #179   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         2294      2308   +14     
  Branches       499       500    +1     
=========================================
+ Hits          2294      2308   +14     

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