Merge rust-bitcoin#2804: Enable getting the witness program from an address

e384f67 Enable getting the witness program from an address (Tobin C. Harding)

Pull request description:

  We have getters for the pubkey hash and script hash but we forgot one for the witness program - add it.

  This is a backport of rust-bitcoin#2796 without the update to api files which don't exist yet on this release branch.

ACKs for top commit:
  apoelstra:
    ACK e384f67

Tree-SHA512: a11cd7cbfcc3b5606651a04395779834a3d70c00c69aef7451f2e8c0321832effe0f1b0ed45e6b132fb9ba7312dcabefa8eb7603f5ab5c66c985a4b01e5c6e1e

Author: apoelstra

bitcoin/src/address/mod.rs

@@ -490,6 +490,16 @@ impl Address {
         }
     }
 
+    /// Gets the witness program for this address if this is a segwit address.
+    pub fn witness_program(&self) -> Option<WitnessProgram> {
+        use AddressInner::*;
+
+        match self.0 {
+            Segwit { ref program, hrp: _ } => Some(*program),
+            _ => None,
+        }
+    }
+
     /// Checks whether or not the address is following Bitcoin standardness rules when
     /// *spending* from this address. *NOT* to be called by senders.
     ///