* Rename parse_str to parse and make it polymorphic.
* Rename parse_utf8 to parse_with, since it is now the callback-based
version of parse
* Add a parse_utf16 method analogous to parse
* Rename existing parse_utf16 method to parse_utf16_with
This brings in the changes from tree-sitter/rust-tree-sitter#5
* 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.
