Crypto fixes/improvements (#2008)

* Crypto fixes:

- Use schnorrkel's HDKD derive
- Assume all key URIs beginning with `/` are prefixed with public
  root phrase.

* Remove commented code.

* Update README

* Update core/primitives/src/ed25519.rs

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

substrate/README.adoc

@@ -259,7 +259,7 @@ We'll start Alice's substrate node first on default TCP port 30333 with her chai
 cargo run --release \-- \
   --base-path /tmp/alice \
   --chain=local \
-  --key Alice \
+  --key //Alice \
   --name "ALICE" \
   --node-key 0000000000000000000000000000000000000000000000000000000000000001 \
   --telemetry-url ws://telemetry.polkadot.io:1024 \
@@ -272,7 +272,7 @@ cargo run --release \-- \
   --base-path /tmp/bob \
   --bootnodes /ip4/127.0.0.1/tcp/30333/p2p/QmQZ8TjTqeDj3ciwr93EJ95hxfDsb9pEYDizUAbWpigtQN \
   --chain=local \
-  --key Bob \
+  --key //Bob \
   --name "BOB" \
   --port 30334 \
   --telemetry-url ws://telemetry.polkadot.io:1024 \