Simplify Orchard Flavor

[ConstanceBeguier]

• Rename OrchardCommonDomain to OrchardPrimitives
• Rename domain folder to primitives
• Replace OrchardFlavor with OrchardCircuit or OrchardPrimitives whenever possible

[PaulLaux]

I like it more then our previous naming.

To conclude:

• can we do somthing with the file primitives.rs? remove or make it relevent
• the file domain.rs exposes functions belong to primitives. can we move it to primitives.rs and remove the file?
• folder domain should be renamed to Primitives? (we can create a new Domain folder inside the renamed Primitives.)
• the struct

#[derive(Debug, Clone)]
pub struct OrchardDomain<P: OrchardPrimitives> {
    /// A parameter needed to generate the nullifier.
    pub rho: Rho,
    phantom: core::marker::PhantomData<P>,
}

should be in zcash_note_encryption_domain.rs or domain.rs not in orchard_primitives.rs

• instead

orchard_primitives::{OrchardDomain, OrchardPrimitives},

we should probable have:

orchard_primitives::{Domain::OrchardDomain, OrchardPrimitives},

consider it, it is inlide with the suggestion above.

[ConstanceBeguier]

I removed primitives.rs file.
Then I moved domain.rs to primitives.rs and I renamed domain folder into primitives.
I moved OrchardDomain struct into the new file primitives/orchard_domain.rs