Add hayoo backend for suggesting imports.

NixOS doesn't seem to have a working hoogle, so hayoo is the better option.  Added a hayoo backend.
I can use cl-mapcan, but I also need to convert from vector to list (using append x nil).

However, I see another problem here.  There is no queuing when asking the user for a response, so when multiple symbols are missing, they seem to be all over each other asking whether to add an import statement.

Some kind of explicit queuing seems to be needed when the prompt is called asyncronously.