Avoid neeless Name => String conversion, interning in ICode
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.
Fast past for ByNameClass in isImpossibleSubtype

These show up often due to the way that searches for
implicit views operate: firstly `A=>B` is sought, and
failing that `(=>A) => B`.
OPT avoid intermediate Name creation
Use _validTo field rather than accessor.

Its showing up in my profiles. Maybe because it is virtual
and overriden in SynchronizedSymbol?