Separate Version impl blocks

In preparation for adding an extension trait; separate the
`transaction::Version` impl blocks into stuff that will stay here and
stuff that will go to `primitives`.

Refactor only, no logic changes.

bitcoin/src/blockdata/transaction.rs

@@ -944,7 +944,9 @@ impl Version {
 
     /// The second Bitcoin transaction version (post-BIP-68).
     pub const TWO: Self = Self(2);
+}
 
+impl Version {
     /// Creates a non-standard transaction version.
     pub fn non_standard(version: i32) -> Version { Self(version) }