Return models, not dicts, from Model.{all,first}

[mesozoic]

Today these methods return dicts, and now they'll return model instances. This one was straightforward but I wrote a couple more tests and dried up a couple others.

Fixes #188.

[codecov[bot]]

Codecov Report

Merging #262 (d1a95e4) into main (4fc4ac0) will increase coverage by 0.38%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
+ Coverage   91.59%   91.97%   +0.38%     
==========================================
  Files          15       15              
  Lines         773      773              
==========================================
+ Hits          708      711       +3     
+ Misses         65       62       -3     

Impacted Files	Coverage Δ
pyairtable/orm/model.py	91.50% <100.00%> (+3.77%)	:arrow_up:

... and 1 file with indirect coverage changes