Fetch Suggestions Asynchronously

[ericfreese]

Will solve a number of performance problems.

Was implemented by @PythonNut using zsh-async in PR #134, but I think we should be able to implement it without adding the dependency.

Seems to me the only real way to make this asynchronous is to have another process that's responsible for fetching the suggestion. zsh-async uses zptys, which might be the best solution. I've also looked a bit into coproc (see gist here), but mostly decided they're not the best way forward.

Next will be looking into zpty.

[PythonNut]

👍 looking forward to a better solution.