Add benchmark script

* Structure `cli` crate as both a library and an executable, so that
benchmarks can import code from the crate.
* Import macros in the Rust 2018 style.

cli/src/tests/parser_api_test.rs

@@ -1,4 +1,5 @@
 use super::helpers::fixtures::get_language;
+use serde_derive::Deserialize;
 use std::thread;
 use tree_sitter::{InputEdit, Language, LogType, Parser, Point, PropertySheet};