Add better coverage of hash functions

[dbosk]

Particularly that collision resistance implies second preimage resistance, which in turn also implies preimage resistance.

The reductions are something along the lines of this:

• col(): x <- rand(), x' <- second_pre(x)
• second_pre(x): y <- H(x), do x' <- pre(y) until x' != x

It's actually a bit more complicated when looking at the details, I can recommend reading "Cryptographic Hash-Function Basics: Definitions, Implications, and Separations for Preimage Resistance, Second-Preimage Resistance, and Collision Resistance" by Rogaway and Shrimpton. They cover the details very nicely.