chore: Split prost-types lib.rs into separate modules. (#1007)

* prost-types: Place type implementations in modules.

* prost-types: Extract modules to separate files.

* prost-types: Move tests to the respective modules.

prost-types/src/any.rs

@@ -0,0 +1,72 @@
+use super::*;
+
+impl Any {
+    /// Serialize the given message type `M` as [`Any`].
+    pub fn from_msg<M>(msg: &M) -> Result<Self, EncodeError>
+    where
+        M: Name,
+    {
+        let type_url = M::type_url();
+        let mut value = Vec::new();
+        Message::encode(msg, &mut value)?;
+        Ok(Any { type_url, value })
+    }
+
+    /// Decode the given message type `M` from [`Any`], validating that it has
+    /// the expected type URL.
+    pub fn to_msg<M>(&self) -> Result<M, DecodeError>
+    where
+        M: Default + Name + Sized,
+    {
+        let expected_type_url = M::type_url();
+
+        match (
+            TypeUrl::new(&expected_type_url),
+            TypeUrl::new(&self.type_url),
+        ) {
+            (Some(expected), Some(actual)) => {
+                if expected == actual {
+                    return Ok(M::decode(self.value.as_slice())?);
+                }
+            }
+            _ => (),
+        }
+
+        let mut err = DecodeError::new(format!(
+            "expected type URL: \"{}\" (got: \"{}\")",
+            expected_type_url, &self.type_url
+        ));
+        err.push("unexpected type URL", "type_url");
+        Err(err)
+    }
+}
+
+impl Name for Any {
+    const PACKAGE: &'static str = PACKAGE;
+    const NAME: &'static str = "Any";
+
+    fn type_url() -> String {
+        type_url_for::<Self>()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn check_any_serialization() {
+        let message = Timestamp::date(2000, 1, 1).unwrap();
+        let any = Any::from_msg(&message).unwrap();
+        assert_eq!(
+            &any.type_url,
+            "type.googleapis.com/google.protobuf.Timestamp"
+        );
+
+        let message2 = any.to_msg::<Timestamp>().unwrap();
+        assert_eq!(message, message2);
+
+        // Wrong type URL
+        assert!(any.to_msg::<Duration>().is_err());
+    }
+}