refactor: simplify parser ?

[juleswritescode]

This is just a suggestion! When looking into the parser, I was sometimes confused by some variable names. Maybe this reduces cognitive load?

1. Some renamings:
   next_pos to current_pos (it's actually current_pos)
   peek() to current() (peek and look_ahead seemed to do the same)

2. Removed eat, since it was only used once

3. Added a is_relevant method which is more readable than !is_irrelevant (to me at least)

and 4. Changed some manual loops to Rust native iterators

Again, just a suggestion – the code was already great, and I don't want to be too invasive 🙌

[psteinroe]

makes sense!