largely stop using RoomCreateEventContent

This became a problem because  #foundation-office:matrix.org has a
malformed create event with its `predecessor` set to a string instead of
a map.

The solution to this is, unfortunately, to do more shotgun parsing to
extract only the desired fields rather than trying to parse the entire
content every time. To prevent this kind of problem from happening
again, `RoomCreateEventContent` must only be used for creating new PDUs,
existing PDUs must be shotgun-parsed.

src/service/rooms/state.rs

@@ -7,9 +7,10 @@ use std::{
 use ruma::{
     api::client::error::ErrorKind,
     events::{
-        room::member::MembershipState, AnyStrippedStateEvent, StateEventType,
-        TimelineEventType,
+        room::{create::PreviousRoom, member::MembershipState},
+        AnyStrippedStateEvent, StateEventType, TimelineEventType,
     },
+    room::RoomType,
     serde::Raw,
     state_res::{self, StateMap},
     EventId, OwnedEventId, OwnedRoomId, RoomId, RoomVersionId, UserId,
@@ -51,6 +52,34 @@ impl ExtractCreateContent for ExtractVersion {
     }
 }
 
+/// Extract the `type` from an `m.room.create` event
+#[derive(Deserialize)]
+pub(crate) struct ExtractType {
+    #[serde(rename = "type")]
+    kind: Option<RoomType>,
+}
+
+impl ExtractCreateContent for ExtractType {
+    type Extract = Option<RoomType>;
+
+    fn extract(self) -> Self::Extract {
+        self.kind
+    }
+}
+
+#[derive(Deserialize)]
+pub(crate) struct ExtractPredecessor {
+    predecessor: Option<PreviousRoom>,
+}
+
+impl ExtractCreateContent for ExtractPredecessor {
+    type Extract = Option<PreviousRoom>;
+
+    fn extract(self) -> Self::Extract {
+        self.predecessor
+    }
+}
+
 pub(crate) struct Service {
     pub(crate) db: &'static dyn Data,
 }