runtime: Bitstring implementation for subtype checking (1/4).

Implement a BitString abstraction for storing nbit-sized characters in a
single machine word (uint32).

This is used to enable an O(1) subtype check in subsequent CLs.

Test: art/test.py -b -j32 --host --target
Bug: 64692057
Change-Id: I792405e0c34242cc1206cedeb18e3e6fb687d94b