Make prefix:<~> actually call .Str on an item
Make prefix:<~> actually call .Str on an item

This fixes the issue that using ~ does not coerce an allomorph into its Str
component. This is in contrast to prefix<+>, which does.

Remove return type constraint, since we don't know what .Str would return
on custom Str subclasses.

Gnouc/rakudo,labster/rakudo,Gnouc/rakudo,softmoth/rakudo,nbrown/rakudo,usev6/rakudo,tbrowder/rakudo,softmoth/rakudo,awwaiid/rakudo,labster/rakudo,usev6/rakudo,tony-o/rakudo,sjn/rakudo,rakudo/rakudo,nbrown/rakudo,jonathanstowe/rakudo,lucasbuchala/rakudo,samcv/rakudo,tony-o/rakudo,salortiz/rakudo,tbrowder/rakudo,labster/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,usev6/rakudo,salortiz/rakudo,tony-o/rakudo,samcv/rakudo,jonathanstowe/rakudo,raydiak/rakudo,softmoth/rakudo,tbrowder/rakudo,nbrown/rakudo,tony-o/rakudo,usev6/rakudo,samcv/rakudo,jonathanstowe/rakudo,Gnouc/rakudo,salortiz/rakudo,awwaiid/rakudo,salortiz/rakudo,tbrowder/rakudo,samcv/rakudo,raydiak/rakudo,nbrown/rakudo,rakudo/rakudo,jonathanstowe/rakudo,lucasbuchala/rakudo,awwaiid/rakudo,labster/rakudo,salortiz/rakudo,tbrowder/rakudo,raydiak/rakudo,tony-o/rakudo,usev6/rakudo,lucasbuchala/rakudo,nbrown/rakudo,labster/rakudo,lucasbuchala/rakudo,rakudo/rakudo,Gnouc/rakudo,labster/rakudo,rakudo/rakudo,softmoth/rakudo,MasterDuke17/rakudo,Gnouc/rakudo,raydiak/rakudo,samcv/rakudo,tony-o/rakudo,sjn/rakudo,salortiz/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,rakudo/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,lucasbuchala/rakudo,Gnouc/rakudo,nbrown/rakudo,sjn/rakudo,sjn/rakudo,awwaiid/rakudo,rakudo/rakudo,jonathanstowe/rakudo,sjn/rakudo,softmoth/rakudo,MasterDuke17/rakudo