Introduce merge(Object), drop compose

[gaku-sei]

The only use case (so far) for the compose decoder is two merged object decoders. It leads to this kind of code:

const decoder = compose(
  object({ foo: bool, bar: str }),
  object({ baz: num }),
);

The main problem here, is that compose will assume that the object decoder will preserve the provided object as is. But the object decoder should probably filter out the "unwanted" attributes.

Changing the behavior of object and dropping compose should be performed in the same PR.