Allow LeSer/BeSer impls missing either Serialize or Deserialize (#655)

* Allow LeSer/BeSer impls missing Serialize/Deserialize

Currently, using `LeSer` or `BeSer` requires that the type implements
both `Serialize` and `DeserializeOwned`, even if we're only using the
trait for one of those functionalities.

Moving the bounds to the methods gives the convenience of the traits
without requiring unnecessary derives.

* Remove unused #[derive(Serialize/Deserialize)]

This should hopefully reduce compile times - if only by a little bit.

Some of these were already unused (we weren't using LeSer/BeSer for the
types), but most are have *become* unused with the change to
LeSer/BeSer.

proxy/src/mgmt.rs

@@ -5,7 +5,7 @@ use std::{
 
 use anyhow::bail;
 use bytes::Bytes;
-use serde::{Deserialize, Serialize};
+use serde::Deserialize;
 use zenith_utils::{
     postgres_backend::{self, query_from_cstring, AuthType, PostgresBackend},
     pq_proto::{BeMessage, SINGLE_COL_ROWDESC},
@@ -60,13 +60,13 @@ struct MgmtHandler {
 //         "Failure": "oops"
 //     }
 // }
-#[derive(Serialize, Deserialize)]
+#[derive(Deserialize)]
 pub struct PsqlSessionResponse {
     session_id: String,
     result: PsqlSessionResult,
 }
 
-#[derive(Serialize, Deserialize)]
+#[derive(Deserialize)]
 pub enum PsqlSessionResult {
     Success(DatabaseInfo),
     Failure(String),