ui: simplify size() to only return width, rename it to term_width()

I'm going to add $COLUMNS override, and it should work even if ioctl() on tty
failed. This means that the return type has to be (Option<u16>, Option<u16>).
Since we don't use the row count, I decided to drop it.

src/progress.rs

@@ -63,8 +63,8 @@ impl Progress {
         }
 
         let bar_width = ui
-            .size()
-            .map(|(cols, _rows)| usize::from(cols))
+            .term_width()
+            .map(usize::from)
             .unwrap_or(0)
             .saturating_sub(self.buffer.len() - control_chars + 2);
         self.buffer.push('[');

src/ui.rs

@@ -288,8 +288,8 @@ impl Ui {
         rpassword::prompt_password(format!("{prompt}: "))
     }
 
-    pub fn size(&self) -> Option<(u16, u16)> {
-        crossterm::terminal::size().ok()
+    pub fn term_width(&self) -> Option<u16> {
+        crossterm::terminal::size().ok().map(|(cols, _)| cols)
     }
 
     /// Construct a guard object which writes `data` when dropped. Useful for