This patch updates the CLI to use anyhow and thiserror for error
management. The main feature that our custom `Error` type was providing
was a _list_ of messages, which would allow us to annotate "lower-level"
errors with more contextual information. This is exactly what's
provided by anyhow's `Context` trait.
(This is setup work for a future PR that will pull the `config` and
`loader` modules out into separate crates; by using `anyhow` we wouldn't
have to deal with a circular dependency between with the new crates.)
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.
