Fix some OkBuffer bugs.

GzipSource exceptions used six hex digits instead of
8 to print ints.

readUtf8 always did an extra copy of the bytes being
read.

Moving bytes between buffers crashed when the destination
was empty and the source was a prefix.

InputStream reading returned values in -128..127 instead
of in 0..255.
Merge pull request #517 from square/jwilson_0208_buffer_fixes

Fix some OkBuffer bugs.