Use level checking for Fresh instances

[odersky]

This PR changes the way we treat fresh instances. Instead of disallowing all of them when instantiating type parameters (as well as in a number of other situations), we allow them but check that inner fresh instances do not get widened to outer ones. This makes code like this one compile

class Ref extends Mutable:
  var x = 0
  def get: Int = x
  mut def put(y: Int): Unit = x = y

class Pair[+X, +Y](val fst: X, val snd: Y)

def twoRefs1(): Pair[Ref^, Ref^] =
  Pair(Ref(), Ref())

Previously, that would not work since the X and Y variables of Pair would be instantiated to Ref^, which was not allowed.

The PR also implements lots of improvements to error diagnostics.

• We now disambiguate different instances of ^, cap, or =>.
• We explain for each such instances what it represents exactly.
• We also turn this on for override errors (not necessarily limited to capture checking).