Don't create ImplicitInfos with implicit class symbols

Implicit classes create a pair of symbols: the class itself and
the implicit factory method. The former should not be included
in the implicit search.
Plug a leak in perRunCaches

I guess we've never leaned to heavily on these, because the
registry of caches was forgetting about most of them
immediately if they compared == at the point of creation.

Consequently, maps like `treatedInfos` in Mixin were holding
onto old types and symbols and leaking memory.
Avoid a memory leak with constructor defaults.

Namers plays host to an elaborate handoff of a tree and a Namer
from the constructor to the companion object into which the
default getters are installed.

This commit nulls out the Namer in the symbol attachment once
we're done with it to avoid holding onto that for all eternity.

We could also remove the attachment altogether, but I'm a
unwilling to touch this code more than needed right now.