add exponentiation algorithm: pow_mod_faster Signed-off-by: makejian <makejian@xiaomi.com>
porting from https://github.com/kokke/tiny-bignum-c commit ac136565378c624365e0f5f556d386b3966bff32 and adapting to the nuttx Signed-off-by: makejian <makejian@xiaomi.com>