executor: add TypedExecutor (#993)

Adds a `TypedExecutor` trait that describes how to spawn futures of a specific
type. This is useful for implementing functions that are generic over an executor
and wish to support both `Send` and `!Send` cases.

Cargo.toml

@@ -21,3 +21,6 @@ members = [
   "tokio-udp",
   "tokio-uds",
 ]
+
+[patch.crates-io]
+tokio-executor = { path = "tokio-executor" }