Add Result.FlatMap

[nightruby-isa]

Would love to have FlatMap option on Result.

Mapping with same error type packing them together.

In bigger Pipelines where the Result on a Map returns a Result itself makes it hard to chain together, especially if you are in an async context.

Result<Foo, Error> CreateFoo(Bar b);
Result<Bar, Error> CreateBar();

CreateBar().Map(b => CreateFoo(b)) // creates Result<Result<Foo, Error>, Error>

CreateBar().FlatMap(b => CreateFoo(b)) // Result<Foo, Error>

[nightruby-isa]

AndThen has the behaviour of a FlatMap