feat: AST pretty-printer

[klkvr]

Closes #27

[DaniPopes]

approach is fine, we can also make individual methods public, eg for expr

[klkvr]

@DaniPopes this should be complete, though no tests yet — unsure if we need those and what'd be the best approach to write them

should we add some helpers to check whether two ASTs are identical?

[TilakMaddy]

I was thinking if there's a method on ast nodes to "peek" into the relevant portion of the source code that would be great too

[DaniPopes]

this should be complete, though no tests yet — unsure if we need those and what'd be the best approach to write them

You can add a -Z flag only used in these tests that re-parses pretty printed code; it should also print it to stdout so it gets checked out in a stdout file
I don't think asserting equality is necessary

I was thinking if there's a method on ast nodes to "peek" into the relevant portion of the source code that would be great too

Please check out the SourceMap APIs, I don't know what this has to do with this PR however