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.

ci/azure-patch-crates.yml

@@ -0,0 +1,16 @@
+steps:
+  - script: |
+      set -e
+
+      # Remove any existing patch statements
+      mv Cargo.toml Cargo.toml.bck
+      sed -n '/\[patch.crates-io\]/q;p' Cargo.toml.bck > Cargo.toml
+
+      # Patch all crates
+      cat ci/patch.toml >> Cargo.toml
+
+      # Print `Cargo.toml` for debugging
+      echo "~~~~ Cargo.toml ~~~~"
+      cat Cargo.toml
+      echo "~~~~~~~~~~~~~~~~~~~~"
+    displayName: Patch Cargo.toml