This patch updates `Store::rooms` from a
`Arc<StdRwLock<BTreeMap<OwnedRoomId, Room>>>` to a
`Arc<StdRwLock<Rooms>>` where `Rooms` is a new type that mimics a
`BTreeMap` but that is observable. It uses an `ObservableVector`
for saving us the costs of writing a new `ObservableMap` type in
`eyeball-im`. It would have too much implications that are clearly
not necessary. The major one being that an `ObservableMap` must emit
`MapDiff`, but we expect `VectorDiff` everywhere in the SDK where rooms
are observable. It would break too many API and too many projects.

The benchmark is coming, but here are the results (for 10'000 rooms,
extreme case):

* `get_rooms` and `get_rooms_filtered` are twice faster (in practise, it
  means 650µs is saved per call),
* `get_room` and `get_or_create_room` are 10% slower (in practise, it
  means 8-10ns is lost per call).

Overall, I believe these results are acceptable, and even an improvement
for the first one.

This patch rewrites `Rooms` to a generic `ObservableMap` struct. It also
adds documentation and tests for this type.

This patch basically implements `ObservableMap::stream` which returns a
batched stream of the values.

This patch implements a new `Store::rooms_stream` method that forwards
the result of `ObservableMap::stream`.

…tion` is enabled.

Running `cargo test -p matrix-sdk-base --features e2e-encryption`
makes the code to fail to compile because `crate::latest_event` isn't
defined. And indeed, it must be defined if `experimental-sliding-sync`
is enabled, but also if `e2e-encryption` is enabled.

This patch implements `Client::rooms_stream`, which forwards the result
of the recently added `Store::rooms_stream` method.

…iding-sync` aren't enabled.

This patch implements `Client::rooms_stream` by forwarding the
result of `matrix_sdk_base::Client::rooms_stream`, and mapping the
`matrix_sdk_base::Room` to `matrix_sdk::Room`.

This patch creates an `ObservableMap` for `wasm32-unknown-unknown` which
simply wraps a `BTreeMap`, and without the `stream` method. Indeed, the
first implementation uses `eyeball_im::ObservableVector` which requires
`Send` and `Sync` on its values, which cannot compile to Wasm.

This patch adds an integration test for `Client::rooms_stream`.