Revert Enum default computation

[corranwebster]

Revert enum defaults to using index 0 except for enum.Enums, which instead use the first value returned from the iterator.

This restores the default behaviour to exactly what it was before the introduction of support for enum.Enum other than special-casing default values for enum.Enum.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b06db02). Click here to learn what that means.
The diff coverage is 77.77%.

@@           Coverage Diff            @@
##             master    #828   +/-   ##
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  Coverage          ?   72.4%           
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  Files             ?      51           
  Lines             ?    6367           
  Branches          ?    1276           
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  Hits              ?    4610           
  Misses            ?    1363           
  Partials          ?     394

Impacted Files	Coverage Δ
traits/trait_types.py	69.73% <100%> (ø)
traits/trait_base.py	61.49% <71.42%> (ø)

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[mdickinson]

LGTM; a couple of nitpicks.

@kitchoi Opinions? Does this seem like a satisfactory resolution to #812 to you?

[corranwebster]

See #835 for a full proposal.

[kitchoi]

Does this seem like a satisfactory resolution to #812 to you?

Yes it does! Thanks both!