Clean up random seed to make a bit more flexible (#2456)

* Reformulate random seed to be more random

- First 80 random values come from cycling the incomplete series (
  instead of filling with zeroes)
- Calculate random material each usage (use a single amalgamated
  ring buffer to store them for avoiding 81 lookups each time)
- Mutate inputs by hashing each with:
  - its index (into the 81)
  - an additional "subject" key provided by caller

This keeps the final output low-influence while still allowing
it to be used as the seed to independent contexts. (Hashing the
result to give the final seed is no better than using parent_hash).

* Docs

* Bump runtime

* Update notes

* Remove feature(alloc)

* Update srml/system/src/lib.rs

Co-Authored-By: gavofyork <github@gavwood.com>

substrate/node/runtime/src/lib.rs

@@ -58,8 +58,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	spec_name: create_runtime_str!("node"),
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
-	spec_version: 71,
-	impl_version: 71,
+	spec_version: 72,
+	impl_version: 72,
 	apis: RUNTIME_API_VERSIONS,
 };