Merge rust-bitcoin/rust-bitcoin#1072: Make opcode::to_u8 a const function

0bae411 Make `opcode::to_u8` a const function (Matt Corallo)

Pull request description:

  In general, if a function can be const, it should be, and this one
  trivially can be, so it should be.

ACKs for top commit:
  tcharding:
    ACK 0bae411
  apoelstra:
    ACK 0bae411

Tree-SHA512: 0df883894ec65dac1b709c581ffc3d2daca737561d05adb971dfef736e4147c59ea66cdc0520296c619e13e1dac70c07cb062b61304541cf488262244f313009

Author: apoelstra

src/blockdata/opcodes.rs

@@ -743,7 +743,7 @@ impl All {
 
     /// Encodes [`All`] as a byte.
     #[inline]
-    pub fn to_u8(self) -> u8 {
+    pub const fn to_u8(self) -> u8 {
         self.code
     }
 }