Speed up storing non-iterables in Buf/Blob
Speed up storing non-iterables in Buf/Blob

Use nqp::push_i instead of .ASSIGN-POS.  This also makes Blob.new(...) work
(spotted while writing tests).

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