upb_inttable() now supports a "compact" operation that will
decide on an array size and put all entries with small enough
keys into the array part for faster lookup.
Also exposed the upb_itof_ent structure and put a few useful
values there, so they are one fewer pointer chase away.
The cost is that a upb_msg will now always have an overhead
of 2*sizeof(void*). This is comparable to proto2 overhead.
The benefit is that upb_msg is now self-describing, and
read-only algorithms can now operate on a upb_msg regardless
of the memory-management scheme.
Also, upb_array and upb_string now know inherently if they
own their associated memory, and upb_array has a generic
pointer for memory management purposes like upb_msg does.
Joshua Haberman