Better referential equality in useField

[Jephuff]

resolves #30
wraps onBlur, onChange, and onFocus in useCallback.
wraps input in useMemo

[codecov]

Codecov Report

Merging #65 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #65   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          83     86    +3     
  Branches       17     17           
=====================================
+ Hits           83     86    +3

Impacted Files	Coverage Δ
src/useField.js	100% <100%> (ø)	⬆️

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