Flow_sedlexing.Utf8
let from_string: string => lexbuf;
let sub_lexeme: lexbuf => int => int => string;
let lexeme: lexbuf => string;
let lexeme_to_buffer: lexbuf => Buffer.t => unit;
This API avoids another allocation
let lexeme_to_buffer2: lexbuf => Buffer.t => Buffer.t => unit;