Rename the coins folder to networks

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

Resolves #357.

networks/ethereum/src/tests/contracts/Schnorr.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: AGPLv3
+pragma solidity ^0.8.0;
+
+import "../../../contracts/Schnorr.sol";
+
+contract TestSchnorr {
+  function verify(
+    bytes32 px,
+    bytes calldata message,
+    bytes32 c,
+    bytes32 s
+  ) external pure returns (bool) {
+    return Schnorr.verify(px, message, c, s);
+  }
+}