using enum dispatch

[samuelcolvin]

Using enum dispatch instead of box.

Some boxes can't be removed.

Seem to give a ~10% speedup over main.

[codecov]

Codecov Report

Merging #41 (07ad2f0) into main (6ebcfda) will decrease coverage by 0.13%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##             main      #41      +/-   ##
==========================================
- Coverage   86.72%   86.59%   -0.14%     
==========================================
  Files          28       28              
  Lines        2238     2313      +75     
==========================================
+ Hits         1941     2003      +62     
- Misses        297      310      +13     

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/validators/any.rs	73.33% <40.00%> (-18.34%)	⬇️
src/validators/function.rs	79.04% <58.06%> (-4.08%)	⬇️
src/validators/model.rs	89.20% <62.50%> (-0.46%)	⬇️
src/validators/dict.rs	70.88% <87.50%> (+0.75%)	⬆️
src/validators/bool.rs	100.00% <100.00%> (ø)
src/validators/float.rs	98.34% <100.00%> (+0.14%)	⬆️
src/validators/int.rs	98.34% <100.00%> (+0.14%)	⬆️
src/validators/list.rs	76.92% <100.00%> (+0.29%)	⬆️
src/validators/literal.rs	95.76% <100.00%> (+0.36%)	⬆️
... and 9 more

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