Fix refinable record fields

[berbiche]

Previously, every record would be refined because the record that was passed to refinable/3 always had no fields.

Gradualizer would then crash on pick_value for types it cannot pick an example value.

Now, the record is looked up in the REnv and every typed field of the record is checked for "refinability".

Do I also need to lookup the records in gradualizer_db at that point (e.g. when checking for the refinement of a remote type)?

[codecov-commenter]

Codecov Report

Merging #338 (0ada97e) into master (e226803) will decrease coverage by 0.01%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   87.46%   87.44%   -0.02%     
==========================================
  Files          18       18              
  Lines        2760     2764       +4     
==========================================
+ Hits         2414     2417       +3     
- Misses        346      347       +1     

Impacted Files	Coverage Δ
src/typechecker.erl	90.25% <80.00%> (-0.04%)	:arrow_down:
src/gradualizer_lib.erl	87.09% <0.00%> (-1.08%)	:arrow_down:
src/gradualizer_fmt.erl	75.35% <0.00%> (+0.70%)	:arrow_up:

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[berbiche]

Found an issue with where pick_value is called with {type, _, any, []}.

I will fix this in a following commit.