Update to use edition 2018

Add `edition = 2018` to the manifest file. No further changes were
necessary to get the code to lint cleanly.

Cargo.toml

@@ -10,6 +10,7 @@ keywords = [ "bitcoin", "bitcoinconsensus", "libbitcoinconsensus" ]
 readme = "README.md"
 exclude = ["/depend/bitcoin/.github/**", "/depend/bitcoin/.tx/**", "/depend/bitcoin/build-aux/**", "/depend/bitcoin/contrib/**", "/depend/bitcoin/depends/**", "/depend/bitcoin/doc/**", "/depend/bitcoin/share/**", "/depend/bitcoin/test/**", "/depend/bitcoin/src/bench/**", "/depend/bitcoin/src/config/**", "/depend/bitcoin/src/consensus/**", "/depend/bitcoin/src/leveldb/**", "/depend/bitcoin/src/policy/**", "/depend/bitcoin/src/qt/**", "/depend/bitcoin/src/rpc/**", "/depend/bitcoin/src/support/**", "/depend/bitcoin/src/test/**", "/depend/bitcoin/src/univalue/**", "/depend/bitcoin/src/wallet/**", "/depend/bitcoin/src/zmq/**"]
 build = "build.rs"
+edition = "2018"
 
 [lib]
 name = "bitcoinconsensus"