dump formulas as s-exprs

[digama0]

Currently, --dump-formula dumps formulas pretty-printed to token sequences, which defeats the purpose of parsing them in the first place. This makes the command more useful by printing the output as a sexpr instead, so it is easier to see the actual parse tree that resulted.

[tirix]

I originally used this --dump-formula as a test for the parser, as well as for the formula renderer, to compare that the dumped formulas were the same as the initial formulas. This was later automatized in grammar::verify.

In this comment it seems @GinoGiotto may have used this function.

MM formulas are easier to read for humans, so there is a use.
S-expressions are of course better suited to understand the inner workings of the grammar parser and are easier to be parsed by a computer (!).

Maybe both modes could cohabit, with a different option for each?

[digama0]

Ping @GinoGiotto. I think given that verify-parse-stmt exists there is very little other use for something that you could accomplish with a sed script.

[GinoGiotto]

I don't need --dump-formula anymore. I was interested in getting a complete list of statements from a database. That is now accomplished by --list-statements #150 so the change in this PR is fine for me.

[tirix]

Ok, then no problem.
Change is minimal and looks good to me.
It does of course expose the inner encoding of the tree, which you are changing in #173