Replace paired and zkcrypto with zexe as the pairing backend for the bls library

[drskalman]

Following discussion with @burdges and experimenting with Zexe library it seems that adapting bls library to Zexe is an easier than to new zkcrypto even only if for a intermediary milestone. Hence this ticket tracks the transition in this library.

[drskalman]

I have added zexe_adapter.rs, to inherit, so to speak in C++ way, CurveAffine and CurveProjective from zexe's ProjectiveCurve and AffineCurve respectively to the missing pieces and properties which EngineBLS expect those curves to implement. Now I'm getting into this problem which I don't really know how to resolve:

error[E0277]: cannot multiply-assign `<Self as ProjectiveCurve>::ScalarField` to `<<Self as CurveProjective>::Affine as AffineCurve>::Projective`
   --> src/zexe_adapter.rs:38:49
    |
38  | ...: CurveAffine<Projective = Self, ScalarField = <Self as ProjectiveCurve>::ScalarField, BaseField = <Self as ProjectiveCurve>::BaseField>
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `<<Self as CurveProjective>::Affine as AffineCurve>::Projective *= <Self as ProjectiveCurve>::ScalarField`
    | 
   ::: /home/klaymen/.cargo/git/checkouts/zexe-f74c42809fa6cfe1/85cac17/algebra-core/src/curves/mod.rs:224:11
    |
224 |         + MulAssign<Self::ScalarField>; // needed due to https://github.com/rust-lang/rust/issues/69640
    |           ---------------------------- required by this bound in `AffineCurve`
    |
    = help: the trait `MulAssign<<Self as ProjectiveCurve>::ScalarField>` is not implemented for `<<Self as CurveProjective>::Affine as AffineCurve>::Projective`
help: consider further restricting the associated type
    |
36  |     + MulAssign<<Self as ProjectiveCurve>::ScalarField> where <<Self as CurveProjective>::Affine as AffineCurve>::Projective: MulAssign<<Self as ProjectiveCurve>::ScalarField>
    |                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[drskalman]

This is done by 3821a36