Various object codecs

[samhh]

Closes #5, #18, #19, #20, and #21. Borrows nomenclature from Serde ^1. Enables decoding the following examples:

type Weather = Sum.Member<"Sun"> | Sum.Member<"Rain", { mm: number }>

// Externally tagged
{ Rain: { mm: 123 } }

// Internally tagged
{ tag: "Rain", mm: 123 }

// Adjacently tagged
{ tag: "Rain", value: { mm: 123 } }

// Untagged
{ mm: 123 }

• [x] I suspect there's a better way to handle nullary. :thinking: And maybe a rename to nullaryCodec for facade disambiguation.