WorkerSolver: Improve memory footprint of shared buffer

[gkjohnson]

• Minimize memory footprints of update list -- maybe store a separate list for links from joints because links take up far less memory than the joints.

• Only copy over what has changed or needs to change (send message to worker with diff?)

• Array could be tightly packed and indices stored for each element

• Goal don't need to take up as much data (#25)

• Sort the links, joints, and goals so they are stored in links, joints, goals order and offsets can be stored for where each starts. Then unnecessary memory can be removed.

Related to #6