tss: Adds RSA Threshold signatures

[jbis9051]

RSA Threshold Signatures

This is an implementation of "Practical Threshold Signatures" by Victor Shoup.
Protocol 1 is implemented.

Threshold Primer

Let l be the total number of players, t be the number of corrupted players, and k be the threshold.
The idea of threshold signatures is that at least k players need to participate to form a valid signature.

Setup consists of a dealer generating l key shares from a key pair and "dealing" them to the players. In this implementation the dealer is trusted.

During the signing phase, at least k players use their key share and the message to generate a signature share.
Finally, the k signature shares are combined to form a valid signature for the message.

Modifications

1. Our implementation is not robust. That is, the corrupted players can prevent a valid signature from being formed by the non-corrupted players. As such, we remove all verification.
2. The paper requires p and q to be safe primes. We do not.

[armfazh]

Looks good in general. Most of the comments are easy to fix.

[armfazh]

Changes look good. Thanks @jbis9051