Create a circomlib-compatible Poseidon gadget

[themighty1]

We need to create a standalone crate which wraps https://github.com/privacy-scaling-explorations/poseidon-gadget/
The Poseidon function should be instantiated with the same params as https://github.com/iden3/circomlib/blob/master/circuits/poseidon.circom
Some tweaks need to be implemented on how inputs and outputs are assigned to the "capacity" and "rate" field elements.

Additionally, in order to be able to compute circomlib Poseidon hashes outside the zk circuit, we need to follow through with this PR privacy-scaling-explorations/poseidon-gadget#5