Add the `deferred` module with a `DeferredField` trait that enables
accumulating multiple unreduced products before performing a single
reduction. The trait uses an accumulation-based API: callers feed
factor pairs into an `Accumulator` via `mul_accumulate` /
`square_accumulate`, then call `reduce` once at the end.
For now, both Fp and Fq implement the trait with `Accumulator = Self`,
performing eager reduction on each accumulation. A later commit will
swap in a wide accumulator that defers reduction for real.
The module is feature-gated behind the `deferred` feature flag.
