Adding Interweaved Type and Term Clauses to function definitions

[Sporarum]

This is no longer up to date, pleaser refer to: #14019

This aims to add the ability to declare functions with many clauses of type parameters, instead of at most one, and to allow those clauses to be interweaved with term clauses, here's a motivating example you can find here:

trait Functor[F[_]]:
  def map[A](x: F[A])[B](f: A => B): F[B]


given Functor[List] with
  def map[A](x: List[A])[B](f: A => B): List[B] =
    x.map(f)

def assertTransformation[F[_]: Functor][A](original: F[A])[B](expected: F[B])(mapping: A => B): Unit =
  assert(expected == summon[Functor[F]].map(original)(mapping))

@main def test =
  assertTransformation(List("a", "b"))(List("a1", "b1")){elt => s"${elt}1"}

The delaying and currying of type parameters allows us to only explicitly use parameters that are harder/impossible to infer, for example:

The exact grammar and semantics can be found in this PR to the scala 2 docs

[soronpo]

A few comments:

• Should handle and test for extension definitions as well.
• Should test of overriding and signature check and proper report when signature does not match.
• I suggest opening a thread on https://contributors.scala-lang.org/ to discuss this language change.