host-sp-messages: don't copy that floppy^H^H^H^H^H^HInventoryData (#2253)

The `InventoryData` enum in `host-sp-messages` is *real* big: at least
512B for the `DimmSpd` message, or perhaps more if one of the other
variants whose fields I didn't want to add up is more than that.
Currently, it `#[derive(Copy)]`s, which feels like a big stack overflow
footgun: if an `InventoryData` value is accidentally moved by value, it
gets copied on the stack. This kills the Hubris.

Thus, this commit just removes the `#[derive(Copy)]`, which should take
the bullets out of the footgun. Other `host-sp-messages` are left alone,
and everything still builds fine. While the `SpToHost` and `HostToSp`
messages also derive `Copy`, they aren't as big, and the `Copy` impl
is necessary because they get stuck in ringbufs.

Author: hawkw

lib/host-sp-messages/src/lib.rs

@@ -292,8 +292,13 @@ impl From<drv_i2c_types::ResponseCode> for InventoryDataResult {
 /// These **cannot be reordered**; the host and SP must agree on them.  New
 /// variants may be added to the end, and existing variants may be extended with
 /// new data (at the end), but no changes should be made to existing bytes.
+// Note: this type may ask you to let it derive `Copy`. DO NOT ALLOW THIS! An
+// `InventoryData` is *really big* --- over 512 bytes --- and deriving `Copy`
+// would make it easy to accidentally pass one by value on the stack, increasing
+// stack usage when it isn't necessary to do so. We would like to only allow
+// this type to be bytewise-copied explicitly by calling `.clone()`, instead.
 #[derive(
-    Debug, Clone, Copy, PartialEq, Eq, Deserialize, Serialize, SerializedSize,
+    Debug, Clone, PartialEq, Eq, Deserialize, Serialize, SerializedSize,
 )]
 #[allow(clippy::large_enum_variant)]
 pub enum InventoryData {