First working writable fluid vec

No compression, a bunch of other restrictions... but a real writable
vector.
Supports variable-length per chunk.
Declared before the MR.invoke call, available as a full-fledge output
vec to feed into the next MR call immediately after the invoke returns.
Simple test does not overwrite text line endings.
Bug fix in binary-search for chunk#.
Shrunk array-length from system keys.