By default, parity-scale-codec does not provide Encode/Decode impls for an owned String.
This is only provided under the "full" feature which is not used by the substrate runtime,
because it should not be used for consensus critical code. So in order for the CompactForm
to be integrated into the substrate runtime, or wherever the "full" feature cannot be used,
then we must parameterize the `String` type so that it can be both an `&'static str` on the
runtime side where it is encoded, and a `String` in client/consuming code where it is decoded.

Add a `compact` member to `Field` to indicate it is `scale_codec::Compact`

…ncoded/decoded as a SCALE Compact type

Handle `codec(skip)` and `codec(index = $int)` attributes

…mpact

Ensure we're working with an outer attribute

Co-authored-by: Andrew Jones <ascjones@gmail.com>

  Better error message
  Remove redundant tests
  Test more enum cases to document index/discriminant interaction