A bunch of bytevector functions

0) `make-bytevector`, `bytevector-length`, `bytevector-u8-ref`,
   `bytevector-u8-set!`, `utf8->string`, `string->utf8`

1)  Added `utf8strlen()`, `utf8memlen()`, `utf8nth`, `utf8memcpy`;
    slightly refactored utf8 routines, added description in comments,
    utf8taillen -> utf8seqlen