fix: parser performance issues

[baszalmstra]

I found a loop that iterates over a lot of elements that accidentally loops O(n) times where it should be O(n). This caused huge performance issues that this MR fixes.

The change looks more complicated than it actually is. What was happening was this:

for _i in 0..events.len() {
        for i in 0..events.len() {
                       // Stuff
        }
}

where it should be:

        for i in 0..events.len() {
                       // Stuff
        }

This caused completions in a large file that I'm using to go from 300ms to 3ms but it affected all operations.

[codecov]

Codecov Report

Merging #307 (0c3e591) into master (9c7e847) will decrease coverage by 0.00%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
- Coverage   81.42%   81.41%   -0.01%     
==========================================
  Files         239      239              
  Lines       14026    14025       -1     
==========================================
- Hits        11420    11419       -1     
  Misses       2606     2606              

Impacted Files	Coverage Δ
crates/mun_syntax/src/parsing/event.rs	96.29% <95.83%> (-0.14%)	⬇️

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