Tidy up Rust RoomVersion structs (#19766)

This is in prep for using the room versions more from Rust.

Main changes:
- Change it so each room version is defined as a delta to the last one.
This is a cosmetic change that makes it easier to ensure the room
version definitions are correct (as they're defined as deltas from
previous versions).
- Move constants to `RoomVersion` constants, like `RoomVersion::V1`, for
convenience.
- Change visibility of various attributes.

Author: erikjohnston

changelog.d/19766.misc

@@ -0,0 +1 @@
+Tidy up Rust `RoomVersion` structs.

rust/src/events/json_object.rs

@@ -34,7 +34,7 @@ use serde::{Deserialize, Serialize};
 /// 1. Fast access of top-level keys (e.g. `event.content["key"]`)
 /// 2. Pure Rust implementation.
 #[derive(Serialize, Deserialize, Clone, Default)]
-#[pyclass(mapping, frozen)]
+#[pyclass(mapping, frozen, skip_from_py_object)]
 #[serde(transparent)]
 pub struct JsonObject {
     object: Arc<BTreeMap<Box<str>, serde_json::Value>>,
@@ -197,7 +197,7 @@ impl JsonObject {
 /// of the object.
 ///
 /// This needs to both be iterable *and* operate like a set.
-#[pyclass(frozen)]
+#[pyclass(frozen, skip_from_py_object)]
 #[derive(Clone)]
 pub struct JsonObjectKeysView {
     object: JsonObject,
@@ -397,7 +397,7 @@ impl JsonObjectKeysView {
 
 /// Helper class returned by `JsonObject.values()` to act as a view into the
 /// values of the object.
-#[pyclass(frozen)]
+#[pyclass(frozen, skip_from_py_object)]
 #[derive(Clone)]
 pub struct JsonObjectValuesView {
     object: JsonObject,
@@ -440,7 +440,7 @@ impl JsonObjectValuesView {
 /// unless the values are unhashable. Since the values are immutable we could
 /// support it, but it's more work and nobody seems to actually use the set
 /// operations on `dict_items` in practice.
-#[pyclass(frozen)]
+#[pyclass(frozen, skip_from_py_object)]
 #[derive(Clone)]
 pub struct JsonObjectItemsView {
     object: JsonObject,