Rewrite class decoder to accepts a decoder for properties

[ilario-pierbattista]

By now the class codec accepts an associative array of key-codec pairs.
It works but has a big problem: how to represent an intersection of properties?

Let's suppose to have

type T = {
  a: string
} & (
  | { b: null, c: null }
  | { b: number, c: boolean }
)

If the class decoder accepted a decoder for properties, then an instance of this type would be decoded using a single decoder, not a union of two similar ones.