Adds `--unpretty` unstable flag, which currently accepts two arguments,
`dot` and `graphdotviz`, which are synonymous, and which prints the
expression we parsed in a graphdotviz-compatible format, i.e. in a form
of a directed graph.
Currently can only parse addition in a right-associative way, i.e.
1 + 2 + 3 => 1 + (2 + 3).
The reason why are we using raw pointers in a form of OffsetStr struct
is because I don't think there is a way to prove to Rust compiler that
tokens the parse_next function returns are always valid references to
the TokenStream source string. Therefore we simply have to bypass the
borrow checker and handle this ourselves.
Aodhnait Étaín