chore: correct handling of unicode in lexer

[ghost]

Description

Using Peekable is not the most efficient choice because its "next" function is slower than the underlying iterator's "next," and calling it is a performance bottleneck.

Problem

Resolves #3334

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [Exceptional Case] Documentation to be submitted in a separate PR.

PR Checklist\

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.