impl AsyncExecutor for tokio::runtime::Handle

[kpreid]

This will allow Tokio-based async benching without necessarily having access to the Runtime itself (e.g. inside of a #[tokio::main] function); a Handle is always obtainable via Handle::current().

[lemmih]

Anyone up for reviewing this PR?

[waywardmonkeys]

@kpreid Could you rebase this forward to fix the merge conflict?

@lemmih Since a tokio::runtime::Handle is just another way to get access to the currently running Runtime, I think this PR looks correct to me (without expert knowledge of tokio).

[kpreid]

I've rebased, but I noticed something weird while working with git on it: the affected files seem to have CRLF line endings (even though the .editorconfig file suggests the repo should be all LF). This is probably just either harmless or something to fix in another PR, but I thought I'd mention it in case I'm missing something.