Fix `map` and `mapErr` types

[rijenkii]

Fixes #616.

Currently functions map and mapErr cannot receive callbacks that return type A | Promise<B>, because the callback is expected to have return type A | Promise<A>.

This PR fixes this, basically changing function signatures from

<A,>(cb: () => A | Promise<A>): A

<A,>(cb: () => A): Awaited<A>