Promise variants of form.validate() and field.validate()

[voxpelli]

Fixes #217

Also:

• async module is removed and replaced with a tiny tiny helper function
• field.validate() now always returns an error, previously it only did so on required errors
• due to the fact that field.validate() rarely returned an error, the functionality of validatePastFirstError was pretty much never respected and the tests was also set up to expect validatePastFirstError to not function. So in reality validatePastFirstError has been active for most cases for most users, so the restoration of it's functionality can almost be a bit breaking 🤔 Any thoughts @ljharb?

Are the tests enough for form.validate()? Its getting late here so I'm running out of time a bit, so I'm opening this PR now anyways so that this code won't be stuck on my computer

To do / decide:

• How to handle a now functional validatePastFirstError
• Update README with these promise variants of .validate() and the one from Add a promised based handle() #215
• Once released, do the same, but for the types at DefinitelyTyped, or bring the types in here? (Bringing them here would be easier as then they can be updated prior to release)

[codecov-io]

Codecov Report

Merging #218 (7a26a2d) into master (86c929d) will increase coverage by 0.99%.
The diff coverage is 88.09%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   56.04%   57.04%   +0.99%     
==========================================
  Files          12       13       +1     
  Lines        1092     1129      +37     
  Branches      268      279      +11     
==========================================
+ Hits          612      644      +32     
- Misses        480      485       +5     

Impacted Files	Coverage Δ
lib/fields.js	97.12% <78.57%> (-2.11%)	⬇️
lib/forms.js	88.65% <81.81%> (-1.00%)	⬇️
lib/async.js	100.00% <100.00%> (ø)

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[ljharb]

I rebased this. I'd like to remove async in its own PR, and tests fail after the first commit, and that's concerning.

[voxpelli]

Rebased it and it passes now

[ljharb]

It was failing between the two commits, but i tweaked the test plan count and it seems good now.