Merge #778: Add from_u8_masked RecoveryId constructor

252053c Add `from_u8_masked` `RecoveryId` constructor (Jamil Lambert, PhD)

Pull request description:

  Add an infallible constructor that takes a u8 and masks off the top 6 bits.

  Mentioned in rust-bitcoin/rust-bitcoin#3965

ACKs for top commit:
  tcharding:
    ACK 252053c
  Kixunil:
    ACK 252053c
  apoelstra:
    ACK 252053c; successfully ran local tests

Tree-SHA512: 698b5289358e39c9cf3137d5291dc08de9bc65c0d36aaad839b9e00e1414a72901c02dd101354422019f9fe396c8c7c4e28465ba38700e67a790c9fa29bd682b

Author: apoelstra

src/ecdsa/recovery.rs

@@ -25,6 +25,19 @@ pub enum RecoveryId {
     Three,
 }
 
+impl RecoveryId {
+    /// Creates a `RecoveryId` from a `u8` value by masking off the top 6 bits.
+    #[inline]
+    pub const fn from_u8_masked(id: u8) -> RecoveryId {
+        match id & 0x03 {
+            0 => RecoveryId::Zero,
+            1 => RecoveryId::One,
+            2 => RecoveryId::Two,
+            _ => RecoveryId::Three,
+        }
+    }
+}
+
 impl TryFrom<i32> for RecoveryId {
     type Error = Error;
     #[inline]