Add DivRemGeneric trait

[0xpantera]

Add DivRemGeneric<T, U> (heterogeneous div-mod) & keep DivRem<T> working

This PR implements the generic, asymmetric division-with-remainder trait that had
been marked TODO in core::traits.
It also supplies a zero-cost compatibility bridge so all existing code that uses the
legacy DivRem<T> continues to compile unchanged.

---

Why introduce DivRemGeneric?

	DivRem<T> (before)	DivRemGeneric<T, U> (now)
Operand types	symmetric (T / T)	fully generic (T / U)
Return types	always (T, T)	associated: (Quotient, Remainder)
Expressiveness	cannot describe u256 / u128	describes any integral mix that has an algorithm
Implementations	hand-written per integer	keep old impls; add only the mixed-width ones you need

Benefits over legacy DivRem<T>

1. Mixed-width math without ad-hoc helpers
   DivRemGeneric<u256, u128> lets us express u256 / u128 directly, instead of exposing u256_safe_div_rem and friends.

2. Accurate associated types
   Call-sites get the exact Quotient / Remainder types from the trait, enabling
   generic algorithms (e.g. wide_div_round) to compile for any (T,U) pair.

3. No breaking changes today
   The bridge keeps DivRem<T> working, so downstream crates can migrate at their
   own pace while getting deprecation nudges.

---

What’s in the patch?

File	Key additions
corelib/src/traits.cairo	• Definition of trait DivRemGeneric<T, U> • #[deprecated] trait DivRem<T> (unchanged API) • Internal module _divrem_bridge that automatically provides DivRemGeneric<T,T> for every pre-existing DivRem<T> impl (u8, u16, u32, u64, u128, u256).
corelib/src/integer.cairo	No behaviour change – existing 29 symmetric impls left intact.
corelib/src/test/divrem_generic_test.cairo	New test-suite exercising: • legacy symmetric path (u32 / u32) • generic asymmetric path (u256 / u128).
corelib/src/lib.cairo	#[feature("generic-divrem")] gate added.

---

Migration / Back-compat

• No user-visible breaking change.
  Code that calls DivRem::<T>::div_rem compiles exactly as before (now routed
  through the bridge).
• New code can opt-in to the generic form by importing core::traits::DivRemGeneric.
• DivRem<T> is soft-deprecated with the attribute:

#[deprecated(feature: "generic-divrem", note: "Use `DivRemGeneric`.")]

The warning only appears when a crate doesn’t enable the new feature flag,
so corelib itself builds cleanly.

[reviewable-StarkWare]

This change is 

[orizi]

Reviewable status: 0 of 7 files reviewed, 1 unresolved discussion

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corelib/src/traits.cairo line 367 at r1 (raw file):

}

/// Performs truncated division **and** remainder.

new traits are not added here - adding traits here is deprecated.

this should probably go to num/traits/ops and should probably additionally lose the Generic in the name.

[0xpantera]

Thanks for the feedback! Will move to 'num/traits/ops'. For future reference, where could I have found that adding traits in 'corelib/src/traits' was deprecated?

[orizi]

it was just an internal decision - you can just see all new traits were added elsewhere.

Reviewable status: 0 of 7 files reviewed, 1 unresolved discussion (waiting on @0xpantera)

[orizi]

Reviewed 1 of 7 files at r2, all commit messages.
Reviewable status: 1 of 11 files reviewed, 3 unresolved discussions (waiting on @0xpantera)

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corelib/src/traits.cairo line 367 at r2 (raw file):

}

/// Legacy trait for division with remainder.

revert the doc changes - this trait is by no means deprecated.
we must make sure the change fully work before such a thing.

---

corelib/src/num/traits/ops/divrem.cairo line 74 at r2 (raw file):

    pub impl BridgeU256 = Bridge<u256>;
}
use _divrem_bridge::*;

1. starting with _ means unused - this is just private.
2. Either impl for all - and therefore no inner module is required, or impl specifically - and then the impl aliases should be out of the module - the use * did both.

Suggestion:

mod by_divrem_legacy {
    /// Generic adapter: if the old symmetric `DivRem<T>` exists,
    /// provide the corresponding `DivRemGeneric<T,T>` implementation.
    impl Impl<T, +crate::traits::DivRem<T>> of super::DivRem<T, T> {
        type Quotient = T;
        type Remainder = T;
        fn div_rem(lhs: T, rhs: NonZero<T>) -> (T, T) {
            crate::traits::DivRem::<T>::div_rem(lhs, rhs)
        }
    }
}
// Instantiate the generic adapter for every concrete integer type
// that already has a symmetric `DivRem` implementation.
pub impl DivRemU8 = by_divrem_legacy::Impl<u8>;
pub impl BridgeU16 = by_divrem_legacy::Impl<u16>;
pub impl BridgeU32 = by_divrem_legacy::Impl<u32>;
pub impl BridgeU64 = by_divrem_legacy::Impl<u64>;
pub impl BridgeU128 = by_divrem_legacy::Impl<u128>;
pub impl BridgeU256 = by_divrem_legacy::Impl<u256>;

[0xpantera]

@orizi Appreciate the feedback. I have reverted the doc changes and fixed the legacy bridge implementation.

[orizi]

Reviewable status: 1 of 25 files reviewed, 4 unresolved discussions (waiting on @0xpantera)

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-- commits line 3 at r3:
fix your rebase - it now contains random stuff.

[0xpantera]

Rebased onto current main, resolved the lowering conflicts by keeping
upstream changes, and rewrote the bridge as requested.
All tests green again. Thanks for your patience.