lang: Refactor discriminator generation

[acheroncrypto]

Problem

Discriminator generation code is repeated in multiple places:

anchor/lang/attribute/account/src/lib.rs

Lines 114 to 120 in 04b2153

	let mut discriminator = [0u8; 8];
	discriminator.copy_from_slice(
	&anchor_syn::hash::hash(discriminator_preimage.as_bytes()).to_bytes()[..8],
	);
	let discriminator: proc_macro2::TokenStream =
	format!("{discriminator:?}").parse().unwrap();
	quote! { &#discriminator }

anchor/lang/attribute/event/src/lib.rs

Lines 40 to 46 in 04b2153

	let discriminator = args.discriminator.unwrap_or_else(|| {
	let discriminator_preimage = format!("event:{event_name}").into_bytes();
	let discriminator = anchor_syn::hash::hash(&discriminator_preimage);
	let discriminator: proc_macro2::TokenStream =
	format!("{:?}", &discriminator.0[..8]).parse().unwrap();
	quote! { &#discriminator }
	});

Summary of changes

Refactor the discriminator generation code to reuse functionality.

The above examples become:

gen_discriminator(namespace, name)

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Note: This PR is part of a greater effort explained in #3097.

[vercel]

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