Make Str.substr-eq between 1.4x and 2.4x faster
Make Str.substr-eq between 1.4x and 2.4x faster

- 2.4x if position in range
- 1.8x if position beyond end of string
- 1.4x if negative position

Position beyond end of string is handled by nqp::eqat, and we also
don't need an nqp::if because we get a Bool in the end anyway.

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