Add data structures for multilinear extensions

[tsunrise]

Description

• Add multilinear extension (evaluation form of multilinear polynomial where the domain is boolean hypercube) data structures in ark-poly crate.
  • create module evaluations, and rename Evaluations as evaluations::univariate::UVEvaluations
  • add trait MultilinearExtension, and two implemetors DenseMultilinearExtension and SparseMultilinearExtension
  • Add dependency hashbrown
• Add related benchmarks in ark-poly-benches
  • Add benchmarks for sparse and dense multilinear extensions

closes: #XXXX

---

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[tsunrise]

Not sure why, but there's some compilation problem related to serialization after I merged #144 today. e.g.:

error[E0276]: impl has stricter requirements than trait
  --> poly\src\domain\radix2.rs:26:64
   |
26 | #[derive(Copy, Clone, Hash, Eq, PartialEq, CanonicalSerialize, CanonicalDeserialize)]
   |                                                                ^^^^^^^^^^^^^^^^^^^^ impl has extra requirement `R: ark_std::io::Read`

and

error[E0277]: the trait bound `W: ark_serialize::Write` is not satisfied
  --> poly\src\domain\general.rs:49:63
   |
49 |             GeneralEvaluationDomain::Radix2(d) => d.serialize(&mut writer),
   |                                                               ^^^^^^^^^^^ the trait `ark_serialize::Write` is not implemented for `W`

(I have not changed anything in domain folder)
Is there any workaround? @weikengchen

[weikengchen]

Let me have a try

[weikengchen]

It seems that the cause is as follows:

• This PR uses ark_std's rand branch.
• The ark-serialize uses the master branch of ark_std.

Is there a reason to use the rand branch? Otherwise, this PR may have a dependency on merging of the rand branch.

[tsunrise]

Thanks! I am using ark-std::{UniformRand, test_rng} (which was only available on rand branch yesterday) because ark_ff::test_rng was marked deprecated. I will use ark_std master branch instead.

[tsunrise]

Hi Ryan, I have added trait MVPolynomialCoefficientForm<F: Field> and MultilinearPolynomialEvaluationForm<F: Field>, and SparsePolynomial implements MVPolynomialCoefficientForm<F: Field>. Does that make sense? @ryanleh

[Pratyush]

LGTM modulo the documentation changes!