Change mutability of `RxToken`'s `consume` argument.

[lmbollen]

This pull request drops the requirement that we can mutate the receive buffer.

I noticed issue #443 already discussed this, but was closed without making the change. Even though the suggestion was to change it.

[lmbollen]

Had to add:

extern crate alloc;
use alloc::vec::Vec;

to fuzz_injector, which should not be a problem since it's for development and not production. @whitequark Feel free to merge or ping me if you still agree with the changes.

[lmbollen]

@whitequark I think we're ready to merge :) But I have no rights.