Fix std feature gating

Add feature gate on "std" to unit tests and the an example.

Cargo.toml

@@ -55,3 +55,4 @@ required-features = ["std"]
 
 [[example]]
 name = "v2-separate-creator-constructor"
+required-features = ["std"]

src/v2/map/input.rs

@@ -839,13 +839,15 @@ mod test {
         // assert_eq!(back.taproot_hash_ty(), Err(InvalidSighashTypeError(nonstd)));
     }
 
+    #[cfg(feature = "std")]
     fn out_point() -> OutPoint {
         let txid = Txid::hash(b"some arbitrary bytes");
         let vout = 0xab;
         OutPoint { txid, vout }
     }
 
     #[test]
+    #[cfg(feature = "std")]
     fn serialize_roundtrip() {
         let input = Input::new(out_point());
 

src/v2/map/output.rs

@@ -346,6 +346,7 @@ impl From<serialize::Error> for InsertPairError {
 }
 
 #[cfg(test)]
+#[cfg(feature = "std")]
 mod tests {
     use super::*;