Crypto: Implement CRT for RSA / BigNum

[calvinmetcalf]

The rsa calculations can be speed up by using the Chinese remainder theorem instead of just a the straight forward modpow. In my own library this caused speeds to more then double. Additionally the inverse of the blind can be calculated more efficiently, switching from something like what you have to the one in bn.js caused performance to more then triple, in other words that one operation was more then twice as costly as all the rest of decryption combined.

[calvinmetcalf]

better link for Chinese remainder theorem

[sirdarckcat]

asked frank/thai/quan if they are interested

[sirdarckcat]

leaving it open for (a)

as per (b) the inverse blind is as-is to keep the algorithm fixed timing.