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.

crypto/transcript/src/lib.rs

@@ -8,7 +8,7 @@ use digest::{
   typenum::{
     consts::U32, marker_traits::NonZero, type_operators::IsGreaterOrEqual, operator_aliases::GrEq,
   },
-  core_api::BlockSizeUser,
+  block_api::BlockSizeUser,
   Digest, Output, HashMarker,
 };