CC: Drop idempotent type maps

[odersky]

Reorganize the code so that maps become more manageable

• drop the idempotent ones and apply immediately
• detect when immediate applications cause errors by freezing observed sets
• restart capture checking with augmented use sets if such errors are detected
• fuse successible SubstBindingMaps and Filters

Hopefully this leads to a more efficient system that avoids the timeouts we have been seeing.

Also, tighten a bunch of rules and checks to rule out non-sensical elements in capture sets.

[natsukagami]

This seems to crash the compiler (should compile fine without captureChecking import)

not really, it's crashing if I turn on tracing for TypeComparer, and it also crashes on main

import language.experimental.captureChecking
import annotation.showAsInfix
import compiletime.*
import compiletime.ops.int.*

enum Tuple:
  case EmptyTuple()
  case `*:`[T, U <: Tuple](x: T, y: U)

object Tuple {
  import Tuple.*
  /** Fold a tuple `(T1, ..., Tn)` into `F[T1, F[... F[Tn, Z]...]]]` */
  type Fold[Tup <: Tuple, Z, F[_, _]] = Tup match
    case EmptyTuple => Z
    case h *: t => F[h, Fold[t, Z, F]]

  type Union[T <: Tuple] = Fold[T, Nothing, [x, y] =>> x | y]

  opaque type Oops[h, t <: Tuple] >: Union[t] = Union[h *: t]
}