Implement Hashable for UUIDv4

[janherich]

This PR adds Hashable typeclass implementation to the UUIDv4 type, together with adding necessary dependency (unordered-collections) to the spago.yaml descriptor.

I first tried to provide the typeclass instance just in my project where I wanted to use UUIDv4 as keys in the HashMap datastructure, but as the UUIDv4 type constructor is not exported (only the type is), I couldn't do it.

[janherich]

@garyb Any update on this ? I would really like to use your UUID implementation, but as I'm using the excellent and very performant purescript-unordered-collections in my project (instead of the more common purescript-ordered-collections), this is a deal breaker.

[garyb]

My main issue with this is it seems a little backwards for this library to gain the dependency - I've used uuidv4 a lot, but as of yet have never used unordered-collections, it seems odd that those would come along with it.

We were discussing adding Hashable to the prelude at one point, and having it derivable by the compiler, but it seems that stalled: purescript/purescript-prelude#188, purescript/purescript#3446