tmp: mark tx_in base_weight as pub

Not intended to merge.  This is a tmp commit in leui of extended txin.
Since I want to make predict_fee available, I can't do this whithout
refactoring effective_value, and I can't do this refactor until
extended txin currently.

bitcoin/src/blockdata/transaction.rs

@@ -229,7 +229,7 @@ impl TxIn {
     /// Returns the input base weight.
     ///
     /// Base weight excludes the witness and script.
-    const BASE_WEIGHT: Weight =
+    pub const BASE_WEIGHT: Weight =
         Weight::from_vb_unwrap(OutPoint::SIZE as u64 + Sequence::SIZE as u64);
 
     /// Returns true if this input enables the [`absolute::LockTime`] (aka `nLockTime`) of its