Extract multi_buffer module out of editor (#3170)

Release Notes:

- N/A

crates/editor/src/display_map/block_map.rs

@@ -993,8 +993,8 @@ mod tests {
     use super::*;
     use crate::display_map::inlay_map::InlayMap;
     use crate::display_map::{fold_map::FoldMap, tab_map::TabMap, wrap_map::WrapMap};
-    use crate::multi_buffer::MultiBuffer;
     use gpui::{elements::Empty, Element};
+    use multi_buffer::MultiBuffer;
     use rand::prelude::*;
     use settings::SettingsStore;
     use std::env;

crates/editor/src/display_map/fold_map.rs

@@ -91,7 +91,7 @@ impl<'a> FoldMapWriter<'a> {
 
             // For now, ignore any ranges that span an excerpt boundary.
             let fold = Fold(buffer.anchor_after(range.start)..buffer.anchor_before(range.end));
-            if fold.0.start.excerpt_id() != fold.0.end.excerpt_id() {
+            if fold.0.start.excerpt_id != fold.0.end.excerpt_id {
                 continue;
             }
 

crates/editor/src/display_map/inlay_map.rs

@@ -1,10 +1,8 @@
-use crate::{
-    multi_buffer::{MultiBufferChunks, MultiBufferRows},
-    Anchor, InlayId, MultiBufferSnapshot, ToOffset,
-};
+use crate::{Anchor, InlayId, MultiBufferSnapshot, ToOffset};
 use collections::{BTreeMap, BTreeSet};
 use gpui::fonts::HighlightStyle;
 use language::{Chunk, Edit, Point, TextSummary};
+use multi_buffer::{MultiBufferChunks, MultiBufferRows};
 use std::{
     any::TypeId,
     cmp,