Set correct collision resistance for Rpo256

[vlopes11]

We are currently defining the collision resistance of Rpo256 as a constant for the type.

However, this will not always be true, as we might drop it to 126-bits, depending on the capacity registers setup.

Ideally, we should split the Rpo256 into two distinct types, and set the collision attribute/capacity setup strategies individually. This way, we would create a better distinction and unmistakable representation of what we should expect from the implementation.

We should evaluate the options we have available and solve the comment below.

Originally posted by @bobbinth in #68 (comment)

[bobbinth]

Closed by #236.