Rename the coins folder to networks

Ethereum isn't a coin. It's a network.

Resolves #357.

networks/monero/io/README.md

@@ -0,0 +1,11 @@
+# Monero IO
+
+Serialization functions, as within the Monero protocol.
+
+This library is usable under no-std when the `std` feature (on by default) is
+disabled.
+
+### Cargo Features
+
+- `std` (on by default): Enables `std` (and with it, more efficient internal
+  implementations).