enable string_slice lint

Also swaps out vendored HTML-escaping code for a dependency that I
imagine has decent testing considering all of its reverse depedencies.

src/utils.rs

@@ -6,7 +6,7 @@ use rand::prelude::*;
 use ring::digest;
 use ruma::{canonical_json::try_from_json_map, CanonicalJsonError, CanonicalJsonObject};
 use std::{
-    cmp, fmt,
+    cmp,
     str::FromStr,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -149,40 +149,3 @@ pub(crate) fn deserialize_from_str<
     }
     deserializer.deserialize_str(Visitor(std::marker::PhantomData))
 }
-
-// Copied from librustdoc:
-// https://github.com/rust-lang/rust/blob/cbaeec14f90b59a91a6b0f17fc046c66fa811892/src/librustdoc/html/escape.rs
-
-/// Wrapper struct which will emit the HTML-escaped version of the contained
-/// string when passed to a format string.
-pub(crate) struct HtmlEscape<'a>(pub(crate) &'a str);
-
-impl fmt::Display for HtmlEscape<'_> {
-    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 HtmlEscape(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;",
-                '\'' => "&#39;",
-                '"' => "&quot;",
-                _ => 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(())
-    }
-}