do not require PhantomData to be imported

[cbgbt]

The state-shift macros currently require the user to have PhantomData in scope. This qualifies the symbols in the macro so that it always uses PhantomData from std and cannot be confused by other symbols that the user may have renamed, for example.

PR Checklist

• Tests
• Documentation

[ozgunozerk]

That's a great suggestion, thanks for pointing that out!

I'll double check if some other changes need to be done in the documentation, etc.

And then we can merge it!

[ozgunozerk]

Would be great if you can also change this line in readme:
_state: (PhantomData), // Don't forget this!
to
_state: (::std::marker::PhantomData), // Don't forget this!,
so that the guidelines would work out of the box.

Will merge it after that!

[ozgunozerk]

Will edit the readme later, let's merge it!