Incorrect indent for multi-byte characters

[t-gebauer]

Multi-byte characters mess up indentation.

(λ foo [:bar
         :baz])

(λ foo [:bar]
        :baz)

> string.len("λ")
2

Same as this issue in parinfer-rust: https://github.com/eraserhd/parinfer-rust/issues/26 (Although parinfer-rust, seems to handle this correctly now.)