Add ?Sized bound to SourceCode to allow casting &str` into it.

[Diegovsky]

I'm trying to cast a borrowed &str to a dyn SourceCode but I'm receiving the error:
error[E0277]: the size for values of type str cannot be known at compilation time
and
help: the trait `Sized` is not implemented for `str`
note: required for the cast from `&str` to `&dyn SourceCode`

Which leads me to Source Code's definition, where it is said to be implemented by str and &str (as expected), but it is not marked as ?Sized which I suspect is the reason why this cast can't be made.

[zkat]

I think this is happening because you’re trying to turn &str into &dyn SourceCode… that is, str (which is unsized, and does NOT implement SourceCode), into dyn SourceCode

try casting &&str to &dyn SourceCode

[Diegovsky]

Well, it is implemented for str.

I couldn't do it with &&str because I store an owned String and I'm trying to provide a SourceCode for Diagnostic::source_code.