perf(buffer): simplify Buffer::filled with macro (#1036)

The `vec![]` macro is highly optimized by the Rust team and shorter.
Don't do it manually.

This change is mainly cleaner code. The only production code that uses
this is `Terminal::with_options` and `Terminal::insert_before` so it's
not performance relevant on every render.

src/buffer/buffer.rs

@@ -55,22 +55,21 @@ pub struct Buffer {
 
 impl Buffer {
     /// Returns a Buffer with all cells set to the default one
+    #[must_use]
     pub fn empty(area: Rect) -> Self {
-        let cell = Cell::default();
-        Self::filled(area, &cell)
+        Self::filled(area, &Cell::default())
     }
 
     /// Returns a Buffer with all cells initialized with the attributes of the given Cell
+    #[must_use]
     pub fn filled(area: Rect, cell: &Cell) -> Self {
         let size = area.area() as usize;
-        let mut content = Vec::with_capacity(size);
-        for _ in 0..size {
-            content.push(cell.clone());
-        }
+        let content = vec![cell.clone(); size];
         Self { area, content }
     }
 
     /// Returns a Buffer containing the given lines
+    #[must_use]
     pub fn with_lines<'a, Iter>(lines: Iter) -> Self
     where
         Iter: IntoIterator,