Result::or : avoid over-specializing the type

Result::or : avoid over-specializing the type

Changes .or() so that it can return a Result with a different E type
than the one it is called on.

Essentially:

    fn or(self, res: Result<T, E>) -> Result<T, E>

becomes

    fn or<F>(self, res: Result<T, F>) -> Result<T, F>

This brings `or` in line with the existing `and` and `or_else` member
types.

This is a
[breaking-change]
Due to some code needing additional type annotations.