Add bimap to Either

[DrKillshot]

Currently we can do stuff like this

Either.fromNullable(aValue, "An error has occurred")
    .map(value => /* Do stuff to the object */),
   .mapError(value => /* Do stuff to the string */)

or

Either.fromNullable(aValue, "An error has occurred")
   .mapError(value => /* Do stuff to the string */)

But in cases where both .map and mapError are present we should be able to do

Either.fromNullable(aValue, "An error has occurred")
    .map(
        obj => /* Do stuff to the object */, // <-- Ok branch
        str => /* Do stuff to the string */,  // <-- Error branch
    )

[DrKillshot]

Same thing could be done for flatmap as well