byte_count(
b1) ->count
Return the number of continuation bytes for a multi-byte UTF-8 character
Given the leading byte of a multi-byte UTF-8 sequence, returns the number
of continuation bytes that follow it: 1 for a 2-byte sequence
(0xC0–0xDF), 2 for a 3-byte sequence (0xE0–0xEF), or 3 for a 4-byte
sequence (0xF0–0xF7). Behaviour is undefined for ASCII or continuation
bytes — call valid_b1 first.