calc_rational
----------------
calc_rational consists of a binary crate calc and a library crate
calc_lib. calc is a CLI calculator that only deals with arithmetic
defined on the rational numbers. Internally it is based on
[`Ratio<T>`](https://docs.rs/num/latest/num/rational/struct.Ratio.html)
and [`BigInt`](https://docs.rs/num-bigint/latest/num_bigint/struct.BigInt.html).
For a formal specification of the calc language, one can read
[lang.pdf](https://git.philomathiclife.com/calc_rational/lang.pdf).