metadata: implement write using encode, not vice versa

metadata: implement write using encode, not vice versa

Using write() for encode() is slower due to the use of BytesIO,
and we never really stream the data out anyway since it is part
of an object that we build, so it's all in memory.

Thus, implement write() using encode() rather than the other
way around.

Together with the previous patches, this speeds up encoding the
metadata by about 54%, and indexing by about 15-20% (my system goes
from ~16.3k paths/sec to ~19.7k paths/sec when all the filesystem
data is already buffered in memory).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>