Refactor parser methods to not return State as part of ParseError

As previously discovered with #4464, it's easy to accidentally mis-use the State value returned on the Err path.

There were mixed assumptions about what that State represents: (1) the State where the error occurred, or (2) the State at the beginning of the thing we were just parsing.

I fixed this up to always mean (2) - at which point we don't actually need to return the State at all - so it's impossible for further discrepency to creep in.

I also took the liberty to refactor a few more methods to be purely combinator-based, rather than calling `parse` directly.

crates/cli/src/format.rs

@@ -157,9 +157,7 @@ fn parse_all<'a>(arena: &'a Bump, src: &'a str) -> Result<Ast<'a>, SyntaxError<'
     let (module, state) = module::parse_header(arena, State::new(src.as_bytes()))
         .map_err(|e| SyntaxError::Header(e.problem))?;
 
-    let (_, defs, _) = module_defs()
-        .parse(arena, state, 0)
-        .map_err(|(_, e, _)| e)?;
+    let (_, defs, _) = module_defs().parse(arena, state, 0).map_err(|(_, e)| e)?;
 
     Ok(Ast { module, defs })
 }