Relocate variable_length_crh from ark-pcd to ark-crypto-primitives #18
Description
In the PCD library, we have implemented the variable-length versions of the CRH in this repo.
These variable-length versions, at a high level, do not fix the length of the input to the CRH. The main idea is that many public parameters of the CRH, which depend on the input length, could be produced by running a seeded, cryptographically secure hash function (e.g., Chacha), modeled as a random oracle.
An example is here: https://github.com/arkworks-rs/pcd/blob/master/src/variable_length_crh/bowe_hopwood/constraints.rs#L42
Thus, it would be useful that this general-purpose variable-length CRH is relocated to ark-crypto-primitives since it ideally would belong here.
Help needed and welcomed!