Added read_utf8($fh, $buf, $length[, $offset]) for reading validated UTF-8 code points from a file handle. Bytes are validated as they are read using a streaming DFA validator, so no PerlIO encoding layer is required on the handle. Ill-formed and truncated sequences are replaced with U+FFFD per the maximal subpart rule and warned in the "utf8" category. Tied file handles are not supported.
Replaced the deprecated utf8n_to_uvuni() with utf8n_to_uvchr() thanks to James E Keenan (@keenan), PR #13.