rustdoc: do not escape quotes in body text

Escaping quote marks is only needed in attributes, not text.

```console
$ du -hs doc-old/ doc-new/
670M    doc-old/
669M    doc-new/
```

src/librustdoc/html/escape.rs

@@ -38,3 +38,39 @@ impl<'a> fmt::Display for Escape<'a> {
         Ok(())
     }
 }
+
+/// Wrapper struct which will emit the HTML-escaped version of the contained
+/// string when passed to a format string.
+///
+/// This is only safe to use for text nodes. If you need your output to be
+/// safely contained in an attribute, use [`Escape`]. If you don't know the
+/// difference, use [`Escape`].
+pub(crate) struct EscapeBodyText<'a>(pub &'a str);
+
+impl<'a> fmt::Display for EscapeBodyText<'a> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Because the internet is always right, turns out there's not that many
+        // characters to escape: http://stackoverflow.com/questions/7381974
+        let EscapeBodyText(s) = *self;
+        let pile_o_bits = s;
+        let mut last = 0;
+        for (i, ch) in s.char_indices() {
+            let s = match ch {
+                '>' => "&gt;",
+                '<' => "&lt;",
+                '&' => "&amp;",
+                _ => continue,
+            };
+            fmt.write_str(&pile_o_bits[last..i])?;
+            fmt.write_str(s)?;
+            // NOTE: we only expect single byte characters here - which is fine as long as we
+            // only match single byte characters
+            last = i + 1;
+        }
+
+        if last < s.len() {
+            fmt.write_str(&pile_o_bits[last..])?;
+        }
+        Ok(())
+    }
+}