Create certain fields from validator functions

[rossmacarthur]

• Add field.create() function to create Field classes from functions.
• Dynamically create Domain, Email, Slug, Url Fields.
• Move validation stuff to a validate module.
• In tests, always import from package not submodules where possible.

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #22   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           7      6    -1     
  Lines         420    437   +17     
=====================================
+ Hits          420    437   +17

Impacted Files	Coverage Δ
serde/field.py	100% <100%> (ø)
serde/validate.py	100% <100%> (ø)

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