Replace `futures` `select` with our own select enum to fix MSRV

Replace `futures` `select` with our own select enum to fix MSRV

`futures` recently broke our MSRV by bumping the `syn` major
version in a patch release. This makes it impractical for us to
use, instead here we replace the usage of its `select_biased` macro
with a trivial enum.

Given its simplicity we likely should have done this without ever
taking the dependency.