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/languages/src/rust.rs

@@ -465,7 +465,6 @@ mod tests {
     use gpui::{BorrowAppContext, Context, Hsla, TestAppContext};
     use language::language_settings::AllLanguageSettings;
     use settings::SettingsStore;
-    use text::BufferId;
     use theme::SyntaxTheme;
 
     #[gpui::test]
@@ -682,8 +681,7 @@ mod tests {
         let language = crate::language("rust", tree_sitter_rust::language());
 
         cx.new_model(|cx| {
-            let mut buffer = Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), "")
-                .with_language(language, cx);
+            let mut buffer = Buffer::local("", cx).with_language(language, cx);
 
             // indent between braces
             buffer.set_text("fn a() {}", cx);