Avoid Vec allocation in Event::data derived implementation

Rather than creating a Vec with the discriminant, then another where
the event object is serialised, to finally merge the two, construct
a temporary `(discriminant, event)` tuple which can be serialised
by borsh in one go.  This avoids the temporary vector allocation and
also simplifies the code.

While at it, make other minor refactoring to the event derive function.

lang/attribute/event/src/lib.rs

@@ -21,12 +21,9 @@ pub fn event(
     let event_name = &event_strct.ident;
 
     let discriminator: proc_macro2::TokenStream = {
-        let discriminator_preimage = format!("event:{event_name}");
-        let mut discriminator = [0u8; 8];
-        discriminator.copy_from_slice(
-            &anchor_syn::hash::hash(discriminator_preimage.as_bytes()).to_bytes()[..8],
-        );
-        format!("{discriminator:?}").parse().unwrap()
+        let discriminator_preimage = format!("event:{event_name}").into_bytes();
+        let discriminator = anchor_syn::hash::hash(&discriminator_preimage);
+        format!("{:?}", &discriminator.0[..8]).parse().unwrap()
     };
 
     let ret = quote! {
@@ -35,9 +32,7 @@ pub fn event(
 
         impl anchor_lang::Event for #event_name {
             fn data(&self) -> Vec<u8> {
-                let mut d = #discriminator.to_vec();
-                d.append(&mut self.try_to_vec().unwrap());
-                d
+                (Self::DISCRIMINATOR, self).try_to_vec().unwrap()
             }
         }
 
@@ -47,13 +42,13 @@ pub fn event(
     };
 
     #[cfg(feature = "idl-build")]
-    {
+    let ret = {
         let idl_build = anchor_syn::idl::build::gen_idl_print_function_for_event(&event_strct);
-        return proc_macro::TokenStream::from(quote! {
+        quote! {
             #ret
             #idl_build
-        });
-    }
+        }
+    };
 
     #[allow(unreachable_code)]
     proc_macro::TokenStream::from(ret)