Make `Str` macroable

[BenRutlandWeb]

the ability to add additional Str methods would be good, I.e. `Str::of('hello')->md5();

[BenRutlandWeb]

The current way Str and Stringable work doesn't allow for the Macroable trait to be added so some though on this is needed as to how calls to Str will store macros on the Stringable object.