Worlds smallest optimization

Preallocate for 8 items in the vec. I chose this value for

1. features
2. description
3. payment hash
4. expire time
5. min_final_cltv
6. payment secret
7. route hint
8. for the memes

Author: benthecarman

lightning-invoice/src/lib.rs

@@ -550,7 +550,7 @@ impl InvoiceBuilder<tb::False, tb::False, tb::False, tb::False, tb::False, tb::F
 			amount: None,
 			si_prefix: None,
 			timestamp: None,
-			tagged_fields: Vec::new(),
+			tagged_fields: Vec::with_capacity(8),
 			error: None,
 
 			phantom_d: core::marker::PhantomData,