runtime: merge multi & single threaded runtimes (#1716)

Simplify Tokio's runtime construct by combining both Runtime variants
into a single type. The execution style can be controlled by a
configuration setting on `Builder`.

The implication of this change is that there is no longer any way to
spawn `!Send` futures. This, however, is a temporary limitation. A
different strategy will be employed for supporting `!Send` futures.

Included in this patch is a rework of `task::JoinHandle` to support
using this type from both the thread-pool and current-thread executors.

0 files changed, 0 insertions, 0 deletions