Group improvements (#28)

* Implement `ConstantTimeEq` for `Group` and `Group::Scalar`

* Changed `expand_message_xmd` `dst` parameter to `GenericArray<u8, D>`

* Remove allocation for `dst_prime` in `expand_message_xmd`

* Remove allocation for `msg_prime` and `b` in `expand_message_xmd`

* Remove unnecessary references in `expand_message_xmd`

* Return `GenericArray` from `expand_message_xmd`

* Oxidize loop in `expand_message_xmd`

* Remove allocation for `b` in `expand_message_xmd`

* Remove allocation for `xor`

* De-duplicate code in `expand_message_xmd`

* Remove unnecessary features

* Make ristretto optional and export all backends

* Make `rand` optional

* Fix wrong documentation

* Document `rand` feature

* Remove empty line

* Fix rustfmt

* Fix testing with other backends

* Iterate by value in `expand_message_xmd`

* Fix typo

* Rename `ristretto_x` features to `ristretto255_x`

* Remove `rand` re-export

* Improve `expand_message_xmd` loop

* Remove unnecessary `#[doc(cfg())]`

* Remove unnecessary `pub(crate)` for `mod p256`

* Fix `p256` only build

* Rename `serialize` feature to `serde`

* Fix missing `Cargo.toml` update

src/tests/mock_rng.rs

@@ -7,7 +7,7 @@
 
 use alloc::vec::Vec;
 use core::cmp::min;
-use rand::{CryptoRng, Error, RngCore};
+use rand_core::{CryptoRng, Error, RngCore};
 
 /// A simple implementation of `RngCore` for testing purposes.
 ///