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/editor/src/display_map.rs

@@ -984,7 +984,6 @@ pub mod tests {
     use settings::SettingsStore;
     use smol::stream::StreamExt;
     use std::{env, sync::Arc};
-    use text::BufferId;
     use theme::{LoadThemes, SyntaxTheme};
     use util::test::{marked_text_ranges, sample_text};
     use Bias::*;
@@ -1445,10 +1444,7 @@ pub mod tests {
 
         cx.update(|cx| init_test(cx, |s| s.defaults.tab_size = Some(2.try_into().unwrap())));
 
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), text)
-                .with_language(language, cx)
-        });
+        let buffer = cx.new_model(|cx| Buffer::local(text, cx).with_language(language, cx));
         cx.condition(&buffer, |buf, _| !buf.is_parsing()).await;
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
 
@@ -1533,10 +1529,7 @@ pub mod tests {
 
         cx.update(|cx| init_test(cx, |_| {}));
 
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), text)
-                .with_language(language, cx)
-        });
+        let buffer = cx.new_model(|cx| Buffer::local(text, cx).with_language(language, cx));
         cx.condition(&buffer, |buf, _| !buf.is_parsing()).await;
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));
 
@@ -1602,10 +1595,7 @@ pub mod tests {
 
         let (text, highlighted_ranges) = marked_text_ranges(r#"constˇ «a»: B = "c «d»""#, false);
 
-        let buffer = cx.new_model(|cx| {
-            Buffer::new(0, BufferId::new(cx.entity_id().as_u64()).unwrap(), text)
-                .with_language(language, cx)
-        });
+        let buffer = cx.new_model(|cx| Buffer::local(text, cx).with_language(language, cx));
         cx.condition(&buffer, |buf, _| !buf.is_parsing()).await;
 
         let buffer = cx.new_model(|cx| MultiBuffer::singleton(buffer, cx));