RsMacroArgument should implement PsiLanguageInjectionHost

[lppedd]

The pre-requisite for injection availability is the macro must accept a TokenStream.

This will allow IntelliLang to automatically propose injection of different languages.
A typical use case might be injecting the Pomsky language to which I'm adding support in IDEA.

At the moment I'm implementing my own injection, which is a lot of work.
So if we can have it in the language itself, it's better.

cc @Aloso

[lppedd]

Beware that currently RsMacroArgument include parenthesis too.

Maybe a strip trailing whitespace filter is also required, or it can be delegated to third parties.