Right now, the strings are not used in comparisons, but they
are passed through the grammar processing pipeline, and are
available to the parse table construction algorithm.
This also cleans up a confusing aspect of the parse table
construction, in which precedences and associativities were
temporarily stored in the parse table data structure itself.
* 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.
