Replace Ciphersuite::hash_to_F

The prior-present `Ciphersuite::hash_to_F` was a sin. Implementations took a
DST, yet were not require to securely handle it. It was also biased towards the
requirements of `modular-frost` as `ciphersuite` was originally written all
those years ago, when `modular-frost` had needs exceeding what `ff`, `group`
satisfied.

Now, the hash is bound to produce an output which can be converted to a scalar
with `ff::FromUniformBytes`. A new `hash_to_F`, which accepts a single argument
of the value to hash (removing the potential to insecurely handle the DST by
removing the DST entirely). Due to `digest` yielding a `GenericArray`, yet
`FromUniformBytes` taking a `const usize`, the `ciphersuite` crate now defines
a `FromUniformBytes` trait taking an array (then implemented for all satisfiers
of `ff::FromUniformBytes`). In order to get the array type from the
`GenericArray`, the output of the hash, `digest` is updated to the `0.11`
release candidate which moves to `flexible-array` which solves that problem.

The existing, specific `hash_to_F` functions have been moved to `modular-frost`
as necessary.

`flexible-array` itself is patched to a fork due to
https://github.com/RustCrypto/hybrid-array/issues/131.

processor/view-keys/Cargo.toml

@@ -7,7 +7,7 @@ repository = "https://github.com/serai-dex/serai/tree/develop/processor/view-key
 authors = ["Luke Parker <lukeparker5132@gmail.com>"]
 keywords = []
 edition = "2021"
-rust-version = "1.73"
+rust-version = "1.85"
 
 [package.metadata.docs.rs]
 all-features = true

processor/view-keys/src/lib.rs

@@ -9,5 +9,5 @@ use ciphersuite::Ciphersuite;
 /// `k` is the index of the key to generate (enabling generating multiple view keys within a
 /// single context).
 pub fn view_key<C: Ciphersuite>(k: u64) -> C::F {
-  C::hash_to_F(b"Serai DEX View Key", &k.to_le_bytes())
+  C::hash_to_F(format!("Serai DEX View Key {k}").as_bytes())
 }