Validation.ofResult doesn't require Semigroup

[gusty]

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