EXC_BAD_ACCESS in Lexer.Cursor.Position.advance()

### Issue Kind Bug ### Source Code ```swift await withDiscardingTaskGroup { taskGroup in for content in fileContents { taskGroup.addTask { // EXC_BAD_ACCESS here, requires `await MainActor.run` // Maybe this isn't meant to be used concurrently, but having to run on `MainActor` is a huge bottleneck. Parser.parse(source: content) } } } ``` ### Description Running into this `EXC_BAD_ACCESS` when running concurrent `Parser.parse`. Crash is in `SwiftParser/Lexer/Cursor.swift`, line 648 (602.0.0) when attempting to `input.popFirst()` inside of `Lexer.Cursor.Position.advance`