Implementation of `CryptographicSponge` for Merlin

[autquis]

Using Merlin, we have the possibility of having a sponge that is not generic on a field.

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[mmagician]

@hdevalence

[autquis]

I had to remove the redundant imports since cargo +nightly check complained. Specifically, Vec is in the prelude and is redundant to import.

[mmagician]

@autquis ah yes see also this PR: https://github.com/arkworks-rs/algebra/pull/790 to tackle the same sort of issues in algebra

Edit: I actually wonder why this works, given that similar changes in poly commit failed no-std

[autquis]

@mmagician I didn't understand what I should fix in arkworks-rs/algebra#790? As it is merged already.

Why it works: I guess because it is not completely no-default-features and r1cs is enabled. This is the command for no_std check:

cargo build --no-default-features --features=r1cs --target aarch64-unknown-none

[mmagician]

What I meant is that the algebra PR addressed similar issues.

[autquis]

A gentle ping :)