Improve PsbtSigHashType conversion methods

Improve the `PsbtSigHashType` conversion methods by doing:

- Re-name `inner` -> `to_u32` as per Rust convention
- Add `from_u32` method

Note, we explicitly do _not_ use suffix 'consensus' because these
conversion methods make no guarantees about the validity of the
underlying `u32`.

src/util/psbt/map/input.rs

@@ -184,8 +184,19 @@ impl PsbtSigHashType {
         }
     }
 
-    /// Obtains the inner sighash byte from this [`PsbtSigHashType`].
-    pub fn inner(self) -> u32 {
+    /// Creates a [`PsbtSigHashType`] from a raw `u32`.
+    ///
+    /// Allows construction of a non-standard or non-valid sighash flag
+    /// ([`EcdsaSigHashType`], [`SchnorrSigHashType`] respectively).
+    pub fn from_u32(n: u32) -> PsbtSigHashType {
+        PsbtSigHashType { inner: n }
+    }
+
+
+    /// Converts [`PsbtSigHashType`] to a raw `u32` sighash flag.
+    ///
+    /// No guarantees are made as to the standardness or validity of the returned value.
+    pub fn to_u32(self) -> u32 {
         self.inner
     }
 }

src/util/psbt/serialize.rs

@@ -193,7 +193,7 @@ impl Deserialize for Vec<u8> {
 
 impl Serialize for PsbtSigHashType {
     fn serialize(&self) -> Vec<u8> {
-        serialize(&self.inner())
+        serialize(&self.to_u32())
     }
 }