Add extension instances

[odersky]

Add a new extension { ... } syntax for extension instances without a collective parameter, and
tweak the translation rules for collective extensions.

Motivation

Collective extensions and normal extension methods are conceptually closely related, but look far apart. The expansion of collective extensions into given instances that implement AnyRef is a cute technical trick, but few would want to write that.

It is generally encouraged that extension methods should be put into given instances since that way they can be often be found in the implicit scope of the left-hand side argument, and do not require an import. Furthermore, it's better to import an implicit extension object instead of all extension methods since that leads to less namespace pollution.

But so far, the only pleasant way to do this was to use collective extensions. These are great where they fit, but they do not fit everything. In particular, one sometimes wants to define a bunch of extension methods that do not share a common left parameter. Then one would have to define a separate collective extension for each extension method, which is also awkward.

Summary

This PR adds extension instances, to address the above mentioned shortcomings. Extension instances are simply extensions without the on part. Example:

extension ops {
  def (xs: Seq[String]).longestStrings: Seq[String] = {
    val maxLength = xs.map(_.length).max
    xs.filter(_.length == maxLength)
  }
  def [T](xs: List[T]).second: T = xs.tail.head
}

This extension groups the two extension methods longestStrings and second into a single given instance ops.

Collective extensions are mapped to extension instances by repeating the common parameter and extension instances are mapped in turn to given instances.

New doc page: https://github.com/lampepfl/dotty/blob/d36f43bc9ae1bc28fd5ade8d1fcfd7d14365301c/docs/docs/reference/contextual/extension-methods.md