Install a lookaside cache to speed up repeated lookups of the same operator

by short-circuiting schema search path and ambiguous-operator resolution
computations.  Remarkably, this buys as much as 45% speedup of repetitive
simple queries that involve operators that are not an exact match to the
input datatypes.  It should be marginally faster even for exact-match
cases, though I've not had success in proving an improvement in benchmark
tests.  Per report from Guillame Smet and subsequent discussion.