Improve the ergonomics of creating local buffers (#10347)

This PR renames `language::Buffer::new` to `language::Buffer::local` and
simplifies its interface. Instead of taking a replica id (which should
always be 0 for the local case) and a `BufferId`, which was awkward and
verbose to construct, it simply takes text and a `cx`.

It uses the `cx` to derive a `BufferId` from the `EntityId` associated
with the `cx`, which should always be positive based on the following
analysis...

We convert the entity id to a u64 using this method on `EntityId`, which
is defined by macros in the `slotmap` crate:

```rust
    pub fn as_ffi(self) -> u64 {
        (u64::from(self.version.get()) << 32) | u64::from(self.idx)
    }
```

If you look at the type of `version` in `KeyData`, it is non-zero:

```rust
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct KeyData {
    idx: u32,
    version: NonZeroU32,
}
```

This commit also adds `Context::reserve_model` and
`Context::insert_model` to determine a model's entity ID before it is
created, which we need in order to assign a `BufferId` in the background
when loading a buffer asynchronously.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/gpui/src/app/model_context.rs

@@ -1,6 +1,7 @@
 use crate::{
     AnyView, AnyWindowHandle, AppContext, AsyncAppContext, Context, Effect, Entity, EntityId,
-    EventEmitter, Model, Subscription, Task, View, WeakModel, WindowContext, WindowHandle,
+    EventEmitter, Model, Reservation, Subscription, Task, View, WeakModel, WindowContext,
+    WindowHandle,
 };
 use anyhow::Result;
 use derive_more::{Deref, DerefMut};
@@ -229,6 +230,18 @@ impl<'a, T> Context for ModelContext<'a, T> {
         self.app.new_model(build_model)
     }
 
+    fn reserve_model<U: 'static>(&mut self) -> Reservation<U> {
+        self.app.reserve_model()
+    }
+
+    fn insert_model<U: 'static>(
+        &mut self,
+        reservation: Reservation<U>,
+        build_model: impl FnOnce(&mut ModelContext<'_, U>) -> U,
+    ) -> Self::Result<Model<U>> {
+        self.app.insert_model(reservation, build_model)
+    }
+
     fn update_model<U: 'static, R>(
         &mut self,
         handle: &Model<U>,
@@ -237,13 +250,6 @@ impl<'a, T> Context for ModelContext<'a, T> {
         self.app.update_model(handle, update)
     }
 
-    fn update_window<R, F>(&mut self, window: AnyWindowHandle, update: F) -> Result<R>
-    where
-        F: FnOnce(AnyView, &mut WindowContext<'_>) -> R,
-    {
-        self.app.update_window(window, update)
-    }
-
     fn read_model<U, R>(
         &self,
         handle: &Model<U>,
@@ -255,6 +261,13 @@ impl<'a, T> Context for ModelContext<'a, T> {
         self.app.read_model(handle, read)
     }
 
+    fn update_window<R, F>(&mut self, window: AnyWindowHandle, update: F) -> Result<R>
+    where
+        F: FnOnce(AnyView, &mut WindowContext<'_>) -> R,
+    {
+        self.app.update_window(window, update)
+    }
+
     fn read_window<U, R>(
         &self,
         window: &WindowHandle<U>,