feat(line): implement iterators for Line (#896)

This allows iterating over the `Span`s of a line using `for` loops and
other iterator methods.

- add `iter` and `iter_mut` methods to `Line`
- implement `IntoIterator` for `Line`, `&Line`, and `&mut Line` traits
- update call sites to iterate over `Line` rather than `Line::spans`

src/buffer/buffer.rs

@@ -243,7 +243,7 @@ impl Buffer {
     pub fn set_line(&mut self, x: u16, y: u16, line: &Line<'_>, width: u16) -> (u16, u16) {
         let mut remaining_width = width;
         let mut x = x;
-        for span in &line.spans {
+        for span in line {
             if remaining_width == 0 {
                 break;
             }