This patch started as an effort to make `time::Timer` private. However, in an
effort to get the build compiling again, more and more changes were made. This
probably should have been broken up, but here we are. I will attempt to
summarize the changes here.
* Feature flags are reorganized to make clearer. `net-driver` becomes
`io-driver`. `rt-current-thread` becomes `rt-core`.
* The `Runtime` can be created without any executor. This replaces `enter`. It
also allows creating I/O / time drivers that are standalone.
* `tokio::timer` is renamed to `tokio::time`. This brings it in line with `std`.
* `tokio::timer::Timer` is renamed to `Driver` and made private.
* The `clock` module is removed. Instead, an `Instant` type is provided. This
type defaults to calling `std::time::Instant`. A `test-util` feature flag can
be used to enable hooking into time.
* The `blocking` module is moved to the top level and is cleaned up.
* The `task` module is moved to the top level.
* The thread-pool's in-place blocking implementation is cleaned up.
* `runtime::Spawner` is renamed to `runtime::Handle` and can be used to "enter"
a runtime context.
A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).
The `timer` implementation is now provided by the main `tokio` crate.
The `timer` functionality may still be excluded from the build by
skipping the `timer` feature flag.
