Immutable selector return types

[elliottkember]

Addresses #737

[elliottkember]

The test:typescript tests were failing this PR, because of the way Readonly<Result> works. If the return type is null or any other type – readonly won't work and throws an error.

My solution here was to copy the Immutable definition from immer and use Immutable<Result> – which solves all the type errors. I don't know whether this is a desired approach (from a license or dependency standpoint) but it made the tests pass which is a good start.