Refactor of scroll and shaping

- Scroll is identified by line index and layout index, instead of just
  layout index
- Shaping has the option to prune, where caches outside of the scroll
  view are cleared
- Syntax editor no longer requires layout of all lines, only of lines
  inside scroll
- BufferLine has a metadata field that can be used by other abstractions
  to know when text was changed

src/lib.rs

@@ -39,7 +39,7 @@
 //! buffer.set_text("Hello, Rust! 🦀\n", attrs, Shaping::Advanced);
 //!
 //! // Perform shaping as desired
-//! buffer.shape_until_scroll();
+//! buffer.shape_until_scroll(true);
 //!
 //! // Inspect the output runs
 //! for run in buffer.layout_runs() {