Deprecate automatic insertion of <a> for <Link>

[rauchg]

It's an anti-pattern. I'd like <Link> to only be an abstract higher-order component that decorates whatever is inside

[tpreusse]

Currently when using styled-components one needs a pretty odd setup:

import NextLink from 'next/prefetch'
import styled from 'styled-components'

const Link = ({children, className, ...props}) => (
  <NextLink {...props}><a className={className}>{children}</a></NextLink>
)

export default styled(Link)`
  color: tomato;
`

Using a styled.a (or anything else that maps href to an a) inside NextLink is not possible because it will fail the child.type test. Maybe remove all the magic and checking completely and add a warning in some other way? Maybe in the onClick handler?

[timneutkens]

@tpreusse please see #816