Rename as_u32 -> to_u32

Rust naming conventions stipulate that conversion methods from owned ->
owned for `Copy` types use the naming convention `to_`.

This change makes the function name objectively better, however it makes
no claims of being the 'best' name. We have had much discussion on using
`to_standard` vs `to_u32` but are unable to reach consensus.

src/blockdata/transaction.rs

@@ -855,8 +855,10 @@ impl EcdsaSigHashType {
         }
     }
 
-    /// Converts to a u32
-    pub fn as_u32(self) -> u32 { self as u32 }
+    /// Converts [`EcdsaSigHashType`] to a `u32` sighash flag.
+    ///
+    /// The returned value is guaranteed to be a valid according to standardness rules.
+    pub fn to_u32(self) -> u32 { self as u32 }
 }
 
 /// Error returned when parsing `SigHashType` fails.

src/util/sighash.rs

@@ -600,7 +600,7 @@ impl<R: Deref<Target=Transaction>> SigHashCache<R> {
         }
 
         self.tx.lock_time.consensus_encode(&mut writer)?;
-        sighash_type.as_u32().consensus_encode(&mut writer)?;
+        sighash_type.to_u32().consensus_encode(&mut writer)?;
         Ok(())
     }