This adds reach capabilities x* as described in the proposal.

Since it is never necessary to write `cap` explicitly, there's no need to treat the
identifier as a soft keyword.

This means that no type parameter can be instantiated with a type that
captures cap covariantly or invariantly in its type.

Two exceptions/special cases:

 - Type arguments for isInstanceOf and asInstanceOf are excluded, they can capture cap
   anywhere.
 - Refining variables in class types can still contain cap since they describe what
   comes from the constructor.

Test reclassifications:

 - i15922.scala was moved to pending. Not clear whether this should compile, and what changes would be necessary to get it there.
 - future-traverse.scala was moved to pending. Not clear how to make this compiler.
 - i15749a.scala was moved to neg. The issue description seems to indicate that the test should not compile, but I am not sure what the outcome should be.

For some reason, http4s in the CB fails compilation otherwise.
It's not clear what the failure has to do with the addition of
$reach to Any, but it goes away when $reach has Method flag.

That way it does not show up in completion suggestions

 - use isTrackableRef everywhere for discrimination (instead of
   just checking the CaptureRef type)
 - streamline treatment of reach refs through `stripReach`

- For i15922, it should not work, as both bad1 and bad2 leak scoped
capabilities. It is moved to neg tests.
- For future-traverse.scala, we could make it work this way. Previously it won't
compile because we instantiated the type parameter of `successful` with a
universal capability, but actually we needn't do that: we could well pick a
local reach capability, as is done in this commit.
- For i15749a.scala, it should work but sadly not right now, due to a limitation
of our current implementation.