Add methods to look up single symbols
Add methods to look up single symbols

Many of the calls to `find_symbol` are with lists that actually only
have one element. Add new single-element versions so those calls can be
optimized, but don't do so yet.

tony-o/rakudo,labster/rakudo,softmoth/rakudo,nbrown/rakudo,MasterDuke17/rakudo,softmoth/rakudo,labster/rakudo,nbrown/rakudo,MasterDuke17/rakudo,labster/rakudo,tony-o/rakudo,salortiz/rakudo,tbrowder/rakudo,tony-o/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,tony-o/rakudo,labster/rakudo,rakudo/rakudo,rakudo/rakudo,tbrowder/rakudo,labster/rakudo,salortiz/rakudo,salortiz/rakudo,softmoth/rakudo,MasterDuke17/rakudo,rakudo/rakudo,rakudo/rakudo,nbrown/rakudo,rakudo/rakudo,rakudo/rakudo,softmoth/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,softmoth/rakudo,tbrowder/rakudo,salortiz/rakudo,MasterDuke17/rakudo,salortiz/rakudo,salortiz/rakudo,tony-o/rakudo,nbrown/rakudo,nbrown/rakudo,nbrown/rakudo,labster/rakudo,tony-o/rakudo,tbrowder/rakudo