Attempts at more terse fixint opt-ins

[pwfff]

Two standalone commits here:

1. An attempt at just making the field attribute a bit more terse, but I think this is basically just the 'wrapper' y'all had before? Still have to annotate every field, plus now I have to use into() everywhere. Diff for my project looks like this:

diff --git a/device/src/messaging/packets.rs b/device/src/messaging/packets.rs
index 0d22758..02c80ab 100644
--- a/device/src/messaging/packets.rs
+++ b/device/src/messaging/packets.rs
@@ -1,4 +1,5 @@
 use arrayref::array_ref;
+use postcard::fixint::*;
 use serde::{Deserialize, Serialize};
 
 use super::wrapper::MAX_MESSAGE_SIZE;
@@ -34,10 +35,8 @@ pub struct RequestControllerCount {}
 
 #[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
 pub struct RequestControllerCountResponse {
-    #[serde(with = "postcard::fixint::le")]
-    packet_id: u32,
-    #[serde(with = "postcard::fixint::le")]
-    controller_count: u32,
+    packet_id: LE<u32>,
+    controller_count: LE<u32>,
 }
 
 impl Handler<RequestControllerCountResponse> for RequestControllerCount {
@@ -48,8 +47,8 @@ impl Handler<RequestControllerCountResponse> for RequestControllerCount {
 
     fn handle(&self, controller: &Controller) -> RequestControllerCountResponse {
         RequestControllerCountResponse {
-            packet_id: 0,
-            controller_count: controller.controller_count(),
+            packet_id: 0.into(),
+            controller_count: controller.controller_count().into(),
         }
     }
 }

2. A macro based on this post I found: https://users.rust-lang.org/t/proc-macro-attribute-to-add-a-serde-deserialize-with-to-every-field-in-a-struct/71231
   This seems... fragile. I did find a serde issue/PR where they wanted to do something similar, but it's old, not very active, and had a lot of feedback I didn't quite grok. This does manage to achieve exactly what I was looking for though, so I have a copy in my own codebase that I'm proceeding with. Changes to my project look like this:

diff --git a/device/src/messaging/packets.rs b/device/src/messaging/packets.rs
index 02c80ab..b7bdeb9 100644
--- a/device/src/messaging/packets.rs
+++ b/device/src/messaging/packets.rs
@@ -1,5 +1,5 @@
 use arrayref::array_ref;
-use postcard::fixint::*;
+use corsairust_macros::all_fields_with;
 use serde::{Deserialize, Serialize};
 
 use super::wrapper::MAX_MESSAGE_SIZE;
@@ -33,10 +33,11 @@ pub trait Handler<R: Serialize> {
 
 pub struct RequestControllerCount {}
 
+#[all_fields_with("postcard::fixint::le")]
 #[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
 pub struct RequestControllerCountResponse {
-    packet_id: LE<u32>,
-    controller_count: LE<u32>,
+    packet_id: u32,
+    controller_count: u32,
 }
 
 impl Handler<RequestControllerCountResponse> for RequestControllerCount {
@@ -47,8 +48,8 @@ impl Handler<RequestControllerCountResponse> for RequestControllerCount {
 
     fn handle(&self, controller: &Controller) -> RequestControllerCountResponse {
         RequestControllerCountResponse {
-            packet_id: 0.into(),
-            controller_count: controller.controller_count().into(),
+            packet_id: 0,
+            controller_count: controller.controller_count(),
         }
     }
 }

There's probably a better way to make this stuff happen at the library level? Like ideally use postcard::fixint::le::* would just magically configure it for your whole module, but that magic is beyond me still 🥲

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[pwfff]

Oh, and this is re: #95