extract AtomEdit and Edit into new ra_text_edit crate

crates/ra_text_edit/src/lib.rs

@@ -0,0 +1,29 @@
+mod edit;
+pub mod text_utils;
+
+pub use crate::edit::{Edit, EditBuilder};
+
+use text_unit::{TextRange, TextUnit};
+
+#[derive(Debug, Clone)]
+pub struct AtomEdit {
+    pub delete: TextRange,
+    pub insert: String,
+}
+
+impl AtomEdit {
+    pub fn replace(range: TextRange, replace_with: String) -> AtomEdit {
+        AtomEdit {
+            delete: range,
+            insert: replace_with,
+        }
+    }
+
+    pub fn delete(range: TextRange) -> AtomEdit {
+        AtomEdit::replace(range, String::new())
+    }
+
+    pub fn insert(offset: TextUnit, text: String) -> AtomEdit {
+        AtomEdit::replace(TextRange::offset_len(offset, 0.into()), text)
+    }
+}