Function from_byte_idx

pub fn from_byte_idx(text: &str, byte_idx: usize) -> usize

Converts from byte-index to utf16-code-unit-index in a string slice.

If the byte is in the middle of a multi-byte char, returns the utf16 index of the char that the byte belongs to.

Any past-the-end index will return the one-past-the-end utf16 index.

Runs in O(N) time.