net: move into tokio crate (#1683)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The `net` implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

Cargo.toml

@@ -5,7 +5,6 @@ members = [
   "tokio-executor",
   "tokio-io",
   "tokio-macros",
-  "tokio-net",
   "tokio-sync",
   "tokio-test",
   "tokio-tls",
@@ -13,5 +12,6 @@ members = [
 
   # Internal
   "examples",
-  "build-tests",
+  "tests-build",
+  "tests-integration",
 ]