runtime: Bitstring implementation for subtype checking (2/4).

Implement the subtype checking label as a stand-alone data type.

This stores the bitstring and overflow bits. The bitstring
contains the encoded (path to root, next) bits.

Test: art/test.py -b -j32 --host --target
Bug: 64692057
Change-Id: I2229e1938f5377d637595ba3f81b8af3464d1ae7