Implement optimal BufferElement packing algorithm for BufferAssignment

[jryans]

We currently just packing BufferElements tightly in the buffer based on the order we encounter them when traversing the constraints.

This strategy minimises wasted bits (bits that the fuzzer mutates that has no impact on the constraints) but it means that BufferElementss are not byte aligned. We now support byte alignment but the packing algorithm is sub-optimal. We need an algorithm to perform optimal packing of BufferElements that is

• Byte aligned
• Minimises wasted bits
• Is stable (i.e. with a fixed random seed we should get the same result every time)