Support fuzzing rich signatures beyond Fuzz(data []byte)

• Add support for fuzzing rich signatures, such as:

func FuzzFunc(re string, input []byte, posix bool) (bool, error)

• Directly manage randomness for rich signatures, which also enables literal injection. This allows examples like this to be guessed within a few seconds of fuzzing:

func FuzzHardToGuessNumber(guessMe int64) {
	if guessMe == 0x123456789 {
		panic("bingo")
	}
}