Now we can also parse unary +, i.e.
+17,
and also expressions that contain it, i.e.
+17 + 23.
We also now have custom Debug implementation for Expression, which
prints them in more useful, s-expression-like syntax, i.e.
+17 + 23 => (+ +17 23).
We also change implementation of `TokenStream`s `next` and `parse_next`
methods to allow to easily implement `peek` method for looking at the
(possible) next token without advancing the stream.
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