[LUR-23] Activate "asm" feature when using Bn256

[huitseeker]

Related : #1018

halo2curves, the provider of the BN256 / Grumpkin cycle has a feature for assembly implementation of field operations on BN256:
https://github.com/privacy-scaling-explorations/halo2curves/blob/main/src/bn256/assembly.rs

This feature is exposed through Arecibo:
https://github.com/lurk-lab/arecibo/blob/85c04b3ab1d8446b0a8337ad738ea11aa94f45b4/Cargo.toml#L112

But not through Lurk. It should be exposed and activated in benchmarks as soon as they use BN256, to ensure we get the best performance.

Caution

this feature can only be enabled on target_arch = x86_64
consider something like:

[target.'cfg(not(target_arch = "x86_64"))'.dependencies]
nova = { ...,  features = [..., "asm"] }
[target.'cfg(target_arch = "x86_64")'.dependencies]
nova = { ..., features = [...] }

_{From SyncLinear.com | LUR-23}