Add functions to handle multiple cregs and bitcounts

[doichanj]

Summary

This PR adds support for measuring and stores to multiple classical registers.
Also adds functions in BitArray class as similar to Python's Qiskit to get bitcount and indexed data access.

Details and comments

There was no function to store to multiple classical registers before. And Sampler returns all the measured bits in a single BitArray data.
Now sampler pub result can return separate BitArray for each classical registers.

BitArray adds new functions. bitcount returns number of 1 bits in each bitstrings.
get_counts can have a list of index which element should be included in the map of counts