Merge #212: hrp len const fn

72ad45a hrp len const fn (Tomáš Zemanovič)

Pull request description:

  makes the `Hrp::len` a const fn

ACKs for top commit:
  clarkmoody:
    ACK 72ad45a
  apoelstra:
    ACK 72ad45a; successfully ran local tests

Tree-SHA512: 8e5181d02f8e85b3b91bb877a2f0afb1561ffd3ec18fabf13da808d59926bea1fe24c8291e078f10a5eecaddab8b5e1f98dee03e84c6e472751d2dd2ceb1d424

src/primitives/hrp.rs

@@ -263,7 +263,7 @@ impl Hrp {
     /// Guaranteed to be between 1 and 83 inclusive.
     #[inline]
     #[allow(clippy::len_without_is_empty)] // HRP is never empty.
-    pub fn len(&self) -> usize { self.size }
+    pub const fn len(&self) -> usize { self.size }
 
     /// Returns `true` if this HRP is valid according to the bips.
     ///