Merge #168: add Height::ZERO associated constants

3ca2a5f add Height::ZERO associated constants (Riccardo Casatta)

Pull request description:

ACKs for top commit:
  apoelstra:
    ACK 3ca2a5f

Tree-SHA512: 2081aa7c395391880e2d82e1b9145682b0323a3dbb848f77ef1ce98d12460078fb1506f865daa6c4a379bd10d39975a3b6371d27a6afbb27b920dff9ff67b878

Author: apoelstra

src/locktime.rs

@@ -304,6 +304,9 @@ impl Decodable for LockTime {
 pub struct Height(u32);
 
 impl Height {
+    /// Height zero
+    pub const ZERO: Self = Height(0);
+
     /// Constructs a new block height.
     ///
     /// # Errors