Switch from widening_mul to carrying_mul

[nazar-pc]

Prepare for upstream API changes: rust-lang/rust#156644

[Copilot]

Pull request overview

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Updates the RISC-V interpreter to use carrying_mul instead of widening_mul to stay compatible with upcoming Rust upstream API changes.

Changes:

• Replaced widening_mul calls with carrying_mul(..., 0) in RV32/RV64 MULH-style paths and a related test.
• Updated crate feature gates to remove widening_mul and add signed_bigint_helpers.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/execution/ab-riscv-interpreter/src/rv64/m/tests.rs	Adjusts mulh test to use carrying_mul API.
crates/execution/ab-riscv-interpreter/src/rv64/m.rs	Swaps widening_mul for carrying_mul in signed×signed high-half multiply.
crates/execution/ab-riscv-interpreter/src/rv32/m.rs	Same API swap as RV64 for 32-bit path.
crates/execution/ab-riscv-interpreter/src/lib.rs	Updates nightly feature gate from widening_mul to signed_bigint_helpers.
crates/execution/ab-riscv-coremark-runner/src/main.rs	Updates feature gate list to remove widening_mul.
crates/execution/ab-riscv-act4-runner/src/main.rs	Updates feature gate list to remove widening_mul.
crates/contracts/core/ab-contract-file/src/lib.rs	Updates feature gate list to remove widening_mul.

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