Rename zcash_wallet to zcash_client_backend, set to 2018 edition

Cargo.lock

@@ -507,6 +507,10 @@ name = "winapi-x86_64-pc-windows-gnu"
 version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "zcash_client_backend"
+version = "0.0.0"
+
 [[package]]
 name = "zcash_primitives"
 version = "0.0.0"
@@ -536,10 +540,6 @@ dependencies = [
  "sapling-crypto 0.0.1",
 ]
 
-[[package]]
-name = "zcash_wallet"
-version = "0.0.0"
-
 [[package]]
 name = "zip32"
 version = "0.0.0"