Feat-string-like-field-decoder

[ThijsBroersen]

This PR should provide support for string-like types (T <: String) to be used as keys in json objects.
The implementation reuses any applicable existing JsonDecoder, e.g. a decoder for a string-based const union.

Foo(aOrB: Map["A" | "B", Int]) derives JsonDecoder

value encoded/decoded

Foo(Map("A" -> 1, "B" -> 2))

json

{"aOrB":{"A":1,"B":2}}

[987Nabil]

@plokhotnyuk can we merge this?

[ThijsBroersen]

rebased and tested it again. All seems OK, mima passes