Refactor: propagate errors in typed_ast

[gertvv]

For issue #6.

This PR replaces most instances of let assert and panic with explicit Result(a, Error) returns. There are two issues of code style that I noticed halfway through and I've adjusted parts of the code I touched to the style I started out with, but let me know if you'd like them changed to the alternative:

1. I prefer to qualify result.try - this feels more natural/readable to me.
2. I'm used to using result.map when it makes sense. You seem to typically use result.try and return an explicit Ok(thing) at the end. I see advantages to the latter style, in particular during refactoring and in longer code blocks.

[gertvv]

I'm not convinced the fix I pushed will make the tests pass. The tests pass for me locally with or without the fix. Any ideas?

[schurhammer]

I'm not convinced the fix I pushed will make the tests pass. The tests pass for me locally with or without the fix. Any ideas?

The tests failed on compiling gig itself to binary - I assume locally you already had the binary available, so this part didn't run. Try deleting the binary and c file between test runs.

Anyway, tests passed :)

[schurhammer]

Looks good, I've left some comments about your TODOs

edit: github doesn't show the line below the TODO comment.. thats annoying

[gertvv]

Thanks, all comments addressed.

[schurhammer]

Awesome, I will merge now. Thank you 💜