SDSL 2.3 (vgteam fork)

Major changes

• Preliminary implementation of rle_vector: a run-length encoded bitvector. The structure may still change in the future.
• Proper semantics for storing a multiset of integers in sd_vector.
• Support for 64-bit ARM.
• Smaller sd_vector by creating only the necessary number of buckets and by choosing the number of buckets / the width of the low parts more carefully.
• int_vector, bit_vector, and sd_vector<> support the simple-sds serialization format.