The `FieldExt` trait was originally the only trait implemented in this
crate. When we added `ff` support, we reworked `FieldExt` to be an
extension trait on top of `ff::PrimeField`, but left the existing impls
in `FieldExt`. This resulted in some circular dependencies that prevent
us from making `FieldExt` conditional (e.g. for no-std support).
This commit removes the cycles like so:
- `ff::PrimeField::{from_repr, to_repr}` were implemented as calls to
`FieldExt::{from_bytes, to_bytes}`. The field encoding/decoding logic
is moved into the `ff::PrimeField` trait impl, and `FieldExt` now
calls into `ff::PrimeField`.
- `ff::Field::sqrt` was implemented in terms of `FieldExt::sqrt_alt`.
Given that the latter is a trivial wrapper around the `SqrtTables`
implementation, we duplicate the call to eliminate the cycle.
- `ff::Field::random` used `FieldExt::from_bytes_wide`, which wraps
either `Fp::from_u512` or `Fq::from_u512`. We now use these internal
methods directly.
