More flexible non-nullary codecs

[samhh]

Nullary sums now have getCodecFromStringlyMappedNullaryTag, but there's nothing equivalent for non-nullary sums if you don't want your data encoded specifically as Serialized.

Conversion functions like getCodecFromMappedNullaryTag, but with one or two supplied per member rather than across all members? How would this be handled in io-ts for non-primitive types?

[samhh]

It'd also be very useful to be able to decode structures like this:

{
  tag: 'MemberName',
  other: 'stuff',
  here: 'too',
}

To:

Sum.Member<'MemberName', { other: string; here: string }>

This may need to operate over entire objects like t.type.

Ideally it can also cover stuff like this:

{ tag: 'Human'; occupation: string } | { tag: 'Cat' }

And the tag needs to be flexible. Sometimes you might need to derive from a boolean property.

This is more complex than anything we already have. Maybe there's a layer of abstraction to discover first that's generalisable beyond sum-types.

[samhh]

Split out into #18, #19, #20, and #21.