Overseer (#1152)

* Initial commit

* Licenses, spaces, docs

* Add a spawner

* Watch spawned subsystems with a FuturesUnordered

* Move the types around a bit

* Suggested fixes by Max

* Add a handler to talk to the Overseer

* FromOverseer and ToOverseer msgs and stopping

* Docs and return errors

* Dont broadcast, have add a from field to messages

* Allow communication between subsystems and outside world

* A message with a oneshot to send result example

* Remove leftover can_recv_msg

* Remove from field from messages

* Dont be generic over stuff

* Gather messages with StreamUnordered

* Fix comments and formatting

* More docs fixes and an example

* Apply suggestions from code review

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

* Fixes from review

Move function from impl block.
Do not panic but resolve with errors if spawner fails or subsystem
resolves.

* Dropping a handler results in a flaky test

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/Cargo.lock

@@ -1144,6 +1144,21 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "femme"
+version = "2.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7b6b21baebbed15551f2170010ca4101b9ed3fdc05822791c8bd4631840eab81"
+dependencies = [
+ "cfg-if",
+ "js-sys",
+ "log 0.4.8",
+ "serde",
+ "serde_derive",
+ "wasm-bindgen",
+ "web-sys",
+]
+
 [[package]]
 name = "file-per-thread-logger"
 version = "0.1.3"
@@ -3284,6 +3299,18 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "77af24da69f9d9341038eba93a073b1fdaaa1b788221b00a69bce9e762cb32de"
 
+[[package]]
+name = "overseer"
+version = "0.1.0"
+dependencies = [
+ "femme",
+ "futures 0.3.5",
+ "futures-timer 3.0.2",
+ "kv-log-macro",
+ "log 0.4.8",
+ "streamunordered",
+]
+
 [[package]]
 name = "owning_ref"
 version = "0.4.1"
@@ -7183,6 +7210,18 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "streamunordered"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9394ee1338fee8370bee649f8a7170b3a56917903a0956467ad192dcf8699ca"
+dependencies = [
+ "futures-core",
+ "futures-sink",
+ "futures-util",
+ "slab",
+]
+
 [[package]]
 name = "string"
 version = "0.2.1"
@@ -8347,6 +8386,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3c7d40d09cdbf0f4895ae58cf57d92e1e57a9dd8ed2e8390514b54a47cc5551"
 dependencies = [
  "cfg-if",
+ "serde",
+ "serde_json",
  "wasm-bindgen-macro",
 ]