Deprecate bitset_vector_iterator_t

[chriso]

Vectors need to be converted to a vector_iterator before they can be used in vector operations. For large vectors this requires allocating memory for potentially millions of individual bitset structures. When profiling large vector operations the allocations/deallocations dominate the time it takes to execute the entire operation.

Deprecate the vector_iterator by:

• Create a bitset_vector_advance function which can be used to step through a vector buffer a single bitset at a time.
• Replace the FOREACH macro with another that accepts bitset_vector_t as a parameter and steps through the buffer using a single temporary bitset allocated on the stack
• Change any vector functions that accept a bitset_vector_iterator_t to instead accept a bitset_vector_t
• Profit

[chriso]

Fixed in the deprecate_iterator branch and merged in e993179