This completes the migration of custom derives to proc-macros, removing
the need for the `custom_derive` feature in consumer code. This commit
also includes documentation, unit tests, and compile UI tests for each
of the derives.
Additionally, this commit improves the existing `FromForm` and
`FromFormValue` derives. The generated code for `FromForm` now returns
an error value indicating the error condition. The `FromFormValue`
derive now accepts a `form` attribute on variants for specifying the
exact value string to match against.
Closes#590.
Closes#670.
Sergio Benitez