Add UnpackElement circuit

[0xjei]

Description

This PR adds the UnpackElement circom template. This circuit allows a single field element to be converted into multiple 50-bit elements.

Compared to the original version of MACI, Anonymous Components were used, resulting in DX improvement and small optimisation.

Related Issue(s)

see #131

Other information

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have run yarn prettier and yarn lint without getting any errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ctrlc03]

Great stuff!!

[0xjei]

The test CI fails because the poseidon-proof zkeys have been updated and the verification keys do not match the new circuit model.