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.

tests-integration/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "tests-integration"
+version = "0.1.0"
+authors = ["Tokio Contributors <[email protected]>"]
+edition = "2018"
+publish = false
+
+[dependencies]
+
+[dev-dependencies]
+tokio = { path = "../tokio" }
+tokio-test = { path = "../tokio-test" }
+
+futures-preview = { version = "=0.3.0-alpha.19", features = ["async-await"] }