Bundle multiple allocations into workspaces

This commit replaces multiple successive allocations from
MLD_ALLOC by a single allocation of a suitable 'workspace'
structure.

This reduces the number of allocations as well as the number
of exit paths, thereby reducing the complexity of CBMC proofs.
It also unifies the zeroization at the end of each function.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker