Some small cleanups (#366)

* No need for version deps in dev-deps

These are all internal so we can drop the version

* Remove wasm-bindgen-cli's parity-wasm dep

No longer needed

* Tweak file hierarchy in webidl tests

Use Cargo's conventions to avoid the need to define `[[test]]` sections

* Remove unused imports

crates/cli/src/bin/wasm2es6js.rs

@@ -2,7 +2,6 @@
 extern crate serde_derive;
 extern crate docopt;
 extern crate failure;
-extern crate parity_wasm;
 extern crate wasm_bindgen_cli_support;
 
 use std::fs::File;