Merge rust-bitcoin#4561: Do not derive Default on CompactTarget

e47836f Do not derive Default on CompactTarget (Tobin C. Harding)

Pull request description:

  It is not immediately obvious what a zero value compact target is. Because of this and the fact that we are trying to release minimal code in `primitives 1.0` its best to remove the `Default` derive.

  This is an API breaking change.

ACKs for top commit:
  apoelstra:
    ACK e47836f; successfully ran local tests

Tree-SHA512: 7edb067a7ed66af8155b86b2391ee839b0a21b3d8d36b555123b14e93f1822b89c080328464b194e4ff47643440c56fa5afee929fe58b52d5f1cf84c9b96ca83

Author: apoelstra

primitives/src/pow.rs

@@ -18,7 +18,7 @@ use core::fmt;
 /// `CompactTarget` and `Target` is lossy *in both directions* (there are multiple `CompactTarget`
 /// values that map to the same `Target` value). Ordering and equality for this type are defined in
 /// terms of the underlying `u32`.
-#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct CompactTarget(u32);