Refactor handling of failures in implicit search

Better encapsulation for `DivergentImplicitRecovery` by replacing
the vars `countDown` and `implicitSym` with a single var holding
`Option[DivergentImplicitTypeError]`.

Also adds a pending test for SI-8460 that will be addressed in the
next commit. This commit is just groundwork and should not change
any results of implicit search.
SI-8460 Fix regression in divergent implicit recovery

Implicit search detects likely cycles by looking at the stack of
open implicits and checking the same implicit appears twice, and
if the second occurrence is trying satisfy an implicit search for
a "dominant" type.

Originally, this condition immediately failed the entire implicit
search. However, since Scala 2.10, this mechanism has been refined to
continue searching after the first divergent implicit is detected.
If a second divergence is found, we fail immediately. If the followup
search fails, we report the first divergence. Otherwise, we
take the successful result.

This mechanism was originally built around exceptions. This proved
to be fragile, and was refactored in SI-7291 / accaa314 to instead
use the `Context.errors` to control the process.

But, since that change, the pattern of implicits in scalanlp/breeze
and Shapeless have been prone to reporting the divergent implicit
errors where they used to recover.

So long as we left the `DivergentImplictTypeError` that originates
from a nested implicit search in `context.errors`, we are unable to
successfully typecheck other candidates. This commit instead
stashes the first such error away in `DivergentImplicitRecovery`,
to clear the way for the alternative path to succeed.

We must retain any other divergent implicit errors, as witnessed by
test/files/neg/t2031.scala, which loops unless we retain divergent
implicit errors that we don't stash in `DivergentImplicitRecovery`.
Refactor: keep DivergentImplicitRecovery logic together.
Refactor rankImplicits, add some more docs
Merge pull request #3669 from adriaanm/rebase-3667

SI-8460 Fix regression in implicit divergence recovery