Update rust lexer

[siddhant-deepsource]

Hey, 👋

In one of the rust files from hexyl/hexyl.rs : src/bin/hexyl.rs , chroma was failing to highlight the lines especially the function around these lines. Here's a screenshot of how these looked like when highlighted by chroma :

After checking the same snippet on pygments (screenshot attached below), it was found that maybe we need to update the rust lexer in chroma w.r.t pygments.

Therefore, I have updated the rust lexer using the pygments2chroma.py script (really handy script btw 💯 ). Please take a look and suggest any changes that may be necessary.

Thanks for this amazing project and the work put into it. 🙌

[siddhant-deepsource]

@alecthomas Have fixed the failing tests, can you please take another look? Thanks.

[alecthomas]

Thanks for contributing!