Simplify ref type

[cknitt]

@rickyvetter as discussed on Discord:

Now that we have records as objects in BS7, let's simplify the type for React refs to:

type ref('value) = {mutable current: 'value};

Ref.t, Ref.current and Ref.setCurrent can be deprecated and removed in a later version.

[flash-gordon]

Just two lines clarifying what the upgrade process looks like:

Accessing current value

Before:

reactRef->React.Ref.current->Js.Nullable.toOption

After

reactRef.current->Js.Nullable.toOption

Type aliases

Before:

type aliasedRef = React.Ref.t(Js.nullable(Dom.element));

After:

type aliasedRef = React.ref(Js.nullable(Dom.element));

Updating ref value

Before:

React.Ref.setCurrent(elRef, None);

After:

elRef.current = None;

I also sent a doc patch.