Gate `rayon` usage behind a feature

[lightsing]

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Describe the feature

There's serval reasons to do this:

• when using alloy-primitives::map::HashMap (eg. std not enabled), the rayon::IntoParallelIterator is not enabled for alloy-primitives::map::HashMap by default, also alloy-primitives did not re-export the hashbrown/rayon feature, user have to enable it via hashbrown crate.
• for zk related usage, parallel is usually not a thing, rayon introduce extra cost here.

Additional context

No response