Change Crossterm backend to write directly to buffer instead of String

Crossterm might actually do WinAPI calls instead of writing ANSI excape
codes so writing to an intermediate String may cause issues on older
versions of Windows. It also fails to compile with Crossterm 0.17.8 due
to Crossterm now expecting the writer to support `flush`, which String
doesn't.

Fixes #373

src/backend/crossterm.rs

@@ -13,10 +13,7 @@ use crossterm::{
     },
     terminal::{self, Clear, ClearType},
 };
-use std::{
-    fmt,
-    io::{self, Write},
-};
+use std::io::{self, Write};
 
 pub struct CrosstermBackend<W: Write> {
     buffer: W,
@@ -52,9 +49,6 @@ where
     where
         I: Iterator<Item = (u16, u16, &'a Cell)>,
     {
-        use fmt::Write;
-
-        let mut string = String::with_capacity(content.size_hint().0 * 3);
         let mut fg = Color::Reset;
         let mut bg = Color::Reset;
         let mut modifier = Modifier::empty();
@@ -62,7 +56,7 @@ where
         for (x, y, cell) in content {
             // Move the cursor if the previous location was not (x - 1, y)
             if !matches!(last_pos, Some(p) if x == p.0 + 1 && y == p.1) {
-                map_error(queue!(string, MoveTo(x, y)))?;
+                map_error(queue!(self.buffer, MoveTo(x, y)))?;
             }
             last_pos = Some((x, y));
             if cell.modifier != modifier {
@@ -70,26 +64,25 @@ where
                     from: modifier,
                     to: cell.modifier,
                 };
-                diff.queue(&mut string)?;
+                diff.queue(&mut self.buffer)?;
                 modifier = cell.modifier;
             }
             if cell.fg != fg {
                 let color = CColor::from(cell.fg);
-                map_error(queue!(string, SetForegroundColor(color)))?;
+                map_error(queue!(self.buffer, SetForegroundColor(color)))?;
                 fg = cell.fg;
             }
             if cell.bg != bg {
                 let color = CColor::from(cell.bg);
-                map_error(queue!(string, SetBackgroundColor(color)))?;
+                map_error(queue!(self.buffer, SetBackgroundColor(color)))?;
                 bg = cell.bg;
             }
 
-            string.push_str(&cell.symbol);
+            map_error(queue!(self.buffer, Print(&cell.symbol)))?;
         }
 
         map_error(queue!(
             self.buffer,
-            Print(string),
             SetForegroundColor(CColor::Reset),
             SetBackgroundColor(CColor::Reset),
             SetAttribute(CAttribute::Reset)
@@ -168,7 +161,7 @@ struct ModifierDiff {
 impl ModifierDiff {
     fn queue<W>(&self, mut w: W) -> io::Result<()>
     where
-        W: fmt::Write,
+        W: io::Write,
     {
         //use crossterm::Attribute;
         let removed = self.from - self.to;