First cut writable fluid-vecs

API is set/set8 mirrors at/at8.
Compressed chunks support 2 new calls: writing into the compressed
chunk, or returning false if the value does not fit, and bulk expanding
the chunk into a NewChunk.
After all bulk writes are done, the chunk needs to be "closed" - which
will recompress NewChunks.
Any writing "opens" the Vec, triggers the wiping out of any cached
rollup data, such as min & max, and it is an error to ask for such data
until the entire Vec is "closed".  Longer term plan is to have MRTask2
do the "close" action (not implemented).
Merge branch 'master' of https://github.com/0xdata/h2o
fix spelling