timer: move tokio-timer into tokio crate (#1674)

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.

tokio-test/tests/block_on.rs

@@ -1,8 +1,9 @@
 #![warn(rust_2018_idioms)]
 
-use std::time::{Duration, Instant};
+use tokio::timer::delay;
 use tokio_test::block_on;
-use tokio_timer::delay;
+
+use std::time::{Duration, Instant};
 
 #[test]
 fn async_block() {