Various API updates and breaking changes

[rossmacarthur]

• Remove extra validation options from built-in type Fields.
• Add min() and max() validation functions.
• Add "inclusive" option to between() validator.
• Do not clutter root namespace with Fields.

[codecov-io]

Codecov Report

Merging #34 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #34   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         502    494    -8     
=====================================
- Hits          502    494    -8

Impacted Files	Coverage Δ
src/serde/model.py	100% <ø> (ø)	⬆️
src/serde/error.py	100% <100%> (ø)	⬆️
src/serde/field.py	100% <100%> (ø)	⬆️
src/serde/__init__.py	100% <100%> (ø)	⬆️
src/serde/validate.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a57bbfc...b5bb64a. Read the comment docs.