BitSet: use 0-based bit-indexing
The 1-based indexing was forcing to promote Int to Int128
in chk_indice & chk_offset, to allow correct computation
with extreme values (e.g. BitSet([typemin(Int)])).
1-based indexing was a left-over from the former BitVector
as underlying implementation.
Switching to 0-based indexing allows both a more-direct mapping
between the model and the implementation, and better
performance.
