Rename the coins folder to networks

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

Resolves #357.

networks/monero/ringct/bulletproofs/README.md

@@ -0,0 +1,14 @@
+# Monero Bulletproofs(+)
+
+Bulletproofs(+) range proofs, as defined by 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).
+- `compile-time-generators` (on by default): Derives the generators at
+  compile-time so they don't need to be derived at runtime. This is recommended
+  if program size doesn't need to be kept minimal.