You normally need not use this method. Use this method only if performance is critical. Returns the bit vector's backing bits.
WARNING: For efficiency reasons and to keep memory usage low,
the array is not copied. So if subsequently you modify the returned array directly via the [] operator, be sure you know what you're doing.
A bitvector is modelled as a long array, i.e. long[] bits holds bits of a bitvector. Each long value holds 64 bits. The i-th bit is stored in bits[i/64] at bit position i % 64 (where bit position 0 refers to the least significant bit and 63 refers to the most significant bit).