Validation.ofResult doesn't require Semigroup

When converting from `Result<'T, 'TError>` to `Validation<'TError, 'T>` we need to either:

 - Make sure `'TError` is a Semigroup (this simply means that the type is additive)
 - Transform to a Semigroup

I'm not sure what's the best way to go, maybe the best backward compatibility way is to add a static constraint to `.ofResult` 