This patch is the first step towards unifying all the thread-local
variables spread out across Tokio. A new `Context` struct is added which
will be used to replace the various thread-locals that exist today.

Initially, `Context` only holds the current runtime handle and the
random number generator. Further PRs will add other thread-local state.

A previous PR removed `runtime::context`. At that time,
`runtime::context` was used as an extra layer to access the various
runtime driver handles. This version of `runtime::context` serves a
different purpose (unifying all the thread-locals).