Ports purescript-proxy into this repo

[JordanMartinez]

Since SProxy is defined in Prelude, we need to port purescript-proxy's Proxy type to here instead now that it can be polykinded.

Note: I assume that IsSymbol should use Proxy now rather than SProxy.

[JordanMartinez]

I'll have to get to this later:

• Update all kind-specific proxy types to use forall proxy. proxy kindValue

[hdgarrood]

I think this should use a type variable proxy instead of Proxy, for the same reason as reflectProxy - that is, so that it works with both Proxy and SProxy until we update it again later.

[JordanMartinez]

Good catch! I've fixed that in latest commits.

[JordanMartinez]

Should the docs for Proxy be updated at all? I think right now it only explains how to use it as a proxy for kind Type, not for any kinds.

[hdgarrood]

I don't think that's necessary; it will be clear that Proxy can be used for any kind once someone has come across the idea of polymorphic kinds, and I think the documentation for polykinds would be a better place to introduce things like the fact that you can use Proxy with any kind.

[JordanMartinez]

Ok. Then I think this PR is ready to be merged if there are no other changes I need to make.