transaction: Add Coinbase newtype to distinguish coinbase transactions

Coinbase transactions have unique consensus rules and exceptions that
distinguish them from regular txs. We represent them using a
dedicated Coinbase(Transaction) new type

We use the transparent_newtype macro from internals for safe reference
conversion (&Transaction -> &Coinbase) without manual unsafe code
also it automatically adds #[repr(transparent)] which guarantees that
Coinbase has the exact same memory layout as Transaction

Author: jrakibi

bitcoin/src/blockdata/transaction.rs

@@ -1154,6 +1154,21 @@ impl InputWeightPrediction {
     }
 }
 
+internals::transparent_newtype! {
+    /// A wrapper type for the coinbase transaction of a block.
+    ///
+    /// This type exists to distinguish coinbase transactions from regular ones at the type level.
+    #[derive(Clone, PartialEq, Eq, Debug, Hash)]
+    pub struct Coinbase(Transaction);
+
+    impl Coinbase {
+        /// Creates a reference to `Coinbase` from a reference to the inner `Transaction`.
+        ///
+        /// This method does not validate that the transaction is actually a coinbase transaction.
+        /// The caller must ensure that the transaction is indeed a valid coinbase transaction
+        pub fn assume_coinbase_ref(inner: &_) -> &Self;
+    }
+}
 mod sealed {
     pub trait Sealed {}
     impl Sealed for super::Transaction {}