Finish no-std port

[davxy]

The crate was not no-std yet.

• missing: #![cfg_attr(not(feature = "std"), no_std)]
• use required std types via ark-std::...
• replace HashSet with BTreeSet.

HashSet is not available in no_std environments. Use BTreeSet instead Ref. https://stackoverflow.com/questions/65469712/what-do-i-replace-vec-and-hashset-with-in-a-no-std-environmement

[swasilyev]

Thank you. I was thinking that we probably don't need prover side no-std, but we can get back to that later when we integrate https://github.com/paritytech/ark-substrate