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/worktree/src/worktree_tests.rs

@@ -21,7 +21,6 @@ use std::{
     path::{Path, PathBuf},
     sync::Arc,
 };
-use text::BufferId;
 use util::{http::FakeHttpClient, test::temp_tree, ResultExt};
 
 #[gpui::test]
@@ -577,11 +576,9 @@ async fn test_open_gitignored_files(cx: &mut TestAppContext) {
     let prev_read_dir_count = fs.read_dir_call_count();
     let buffer = tree
         .update(cx, |tree, cx| {
-            tree.as_local_mut().unwrap().load_buffer(
-                BufferId::new(1).unwrap(),
-                "one/node_modules/b/b1.js".as_ref(),
-                cx,
-            )
+            tree.as_local_mut()
+                .unwrap()
+                .load_buffer("one/node_modules/b/b1.js".as_ref(), cx)
         })
         .await
         .unwrap();
@@ -621,11 +618,9 @@ async fn test_open_gitignored_files(cx: &mut TestAppContext) {
     let prev_read_dir_count = fs.read_dir_call_count();
     let buffer = tree
         .update(cx, |tree, cx| {
-            tree.as_local_mut().unwrap().load_buffer(
-                BufferId::new(1).unwrap(),
-                "one/node_modules/a/a2.js".as_ref(),
-                cx,
-            )
+            tree.as_local_mut()
+                .unwrap()
+                .load_buffer("one/node_modules/a/a2.js".as_ref(), cx)
         })
         .await
         .unwrap();