Impose a configurable limit on the total number of nodes constructed during an implicit search.

Fixes scala#13838

 - show what the root query was
 - under -explain, show the trace until the overflow occurred.

Half the limit so that we fail in a bit less than a minute instead of more than 5 minutes.

 - Use the wildcard approximation instead of the original type
   since that one determined what is eligible, and the goal
   is to refuse the search if everything is eligible.
 - Also refuse underspecified implicit parameters, not just
   conversions.
 - Treat wildcard types as underspecified.

Two tests had to be reclassified. But the original tests were not meant to compile
anyway. They were bout misleading error messages (no longer the case) and crashers.

Make another test to exercise the original large search behavior