Allow setting drfuzz iteration count implicitly based on buffer size with exhaustive mutator

Currently, we allow user to specify the maximum number of repeated iterations for a fuzzing test. However for different input buffer, we may want to have different mutation iterations. One way is to tell maximum number of bits to flip instead of fixed number of iterations. For example, for max 1 bit to flip, a 16-byte input buffer will be repeated `16x8` times, while a 32-byte input buffer will be repeated `32x8` times. For both cases, each bit is flipped once.