default all feature flags to off (#1811)

Changes the set of `default` feature flags to `[]`. By default, only
core traits are included without specifying feature flags. This makes it
easier for users to pick the components they need.

For convenience, a `full` feature flag is included that includes all
components.

Tests are configured to require the `full` feature. Testing individual
feature flags will need to be moved to a separate crate.

Closes #1791

tests-integration/Cargo.toml

@@ -6,9 +6,10 @@ edition = "2018"
 publish = false
 
 [dependencies]
+tokio = { path = "../tokio", features = ["full"] }
+doc-comment = "0.3.1"
 
 [dev-dependencies]
-tokio = { path = "../tokio" }
 tokio-test = { path = "../tokio-test" }
 
 futures = { version = "0.3.0", features = ["async-await"] }

tests-integration/src/lib.rs

@@ -0,0 +1,4 @@
+use doc_comment::doc_comment;
+
+// #[doc = include_str!("../../README.md")]
+doc_comment!(include_str!("../../README.md"));