Field with deserialize_with should not implement Deserialize

[dtolnay]

Addresses #259.

The solution is, if a field is missing:

• If there is a default expression, use the default expression (this includes skip_deserializing).
• If the field has deserialize_with, fail with Visitor::Error::missing_field(name).
• Otherwise call visitor.missing_field(name) to possibly handle the field.

This means deserialize_with no longer goes through the visitor's missing_field, instead failing directly unless there is a default. Thus fields with deserialize_with do not require Deserialize as a bound. The missing field behavior can be controlled by setting a default for the field.

[dtolnay]

The Travis failure is due to #313.

[dtolnay]

Rebased, per #315 (comment).

[oli-obk]

took me a while to grok it, but the implementation is sound.

I'm fine with the design, and can't think of another one until specialization stabilizes (so missing_field can handle fields that don't implement Deserialize)