Optimize <Link>s when active-apathetic

[pshrmn]

Now that the context.location has been removed in favor of <LocationSubscriber> I believe that it would be useful to know when not to subscribe and optimize for those situations. The only <Link> components that need to know location information are ones that care about whether or not they are active.

// care when:
// activeStyle is provided
<Link activeStyle={{color: 'blue'}} to='/foo'>Foo</Link>
// activeClassName is provided
<Link activeClassName='active' } to='/bar'>Bar</Link>
// children is a function
<Link to='/seuss'>
  {
    ({isActive, ...rest}) => (
      <a {...rest} className={isActive ? 'one fish' : 'two fish'}>
        { isActive ? 'red fish' : 'blue fish' }
      </a>
    )
  }
</Link>

// doesn't care
<Link to='/quux'>Quux</Link>

I believe that those are the only cases where location information is necessary. When none of those three cases are true, the <Link> should be able to return an <a> that is not wrapped with a <LocationSubscriber>.

[timdorr]

This is coming out of this conversation: #3961 (comment)

I also agree on this. A PR would probably be best for this kind of thing.

[ryanflorence]

let's continue discussion on the PR