UTF-8 string, Yconv() converts it into an octal sequence. If the
string converted to more than 30 bytes, the str buffer would
overflow. For example, 4 Greek runes became 32 bytes, 3 Hiragana
runes became 36 bytes, and 2 Gothic runes became 32 bytes. In
8l, 6l and 5l the function is Sconv(). For some reason, only 5l uses
the constant STRINGSZ (defined as 200) for the buffer size.
R=rsc
https://golang.org/cl/168045
