Remove dependency on StringReader from rustdoc highlighter

rustc_lexer is the lossless lexer, which is a better fit for
approximate syntax highlighting.

As a side-effect, we can now syntax-highlight even broken code.

src/test/rustdoc/bad-codeblock-syntax.rs

@@ -1,33 +1,33 @@
 // @has bad_codeblock_syntax/fn.foo.html
-// @has - '//*[@class="docblock"]/pre/code' '\_'
+// @has - '//*[@class="docblock"]' '\_'
 /// ```
 /// \_
 /// ```
 pub fn foo() {}
 
 // @has bad_codeblock_syntax/fn.bar.html
-// @has - '//*[@class="docblock"]/pre/code' '`baz::foobar`'
+// @has - '//*[@class="docblock"]' '`baz::foobar`'
 /// ```
 /// `baz::foobar`
 /// ```
 pub fn bar() {}
 
 // @has bad_codeblock_syntax/fn.quux.html
-// @has - '//*[@class="docblock"]/pre/code' '\_'
+// @has - '//*[@class="docblock"]' '\_'
 /// ```rust
 /// \_
 /// ```
 pub fn quux() {}
 
 // @has bad_codeblock_syntax/fn.ok.html
-// @has - '//*[@class="docblock"]/pre/code[@class="language-text"]' '\_'
+// @has - '//*[@class="docblock"]' '\_'
 /// ```text
 /// \_
 /// ```
 pub fn ok() {}
 
 // @has bad_codeblock_syntax/fn.escape.html
-// @has - '//*[@class="docblock"]/pre/code' '\_ <script>alert("not valid Rust");</script>'
+// @has - '//*[@class="docblock"]' '\_ <script>alert("not valid Rust");</script>'
 /// ```
 /// \_
 /// <script>alert("not valid Rust");</script>
@@ -35,7 +35,7 @@ pub fn ok() {}
 pub fn escape() {}
 
 // @has bad_codeblock_syntax/fn.unterminated.html
-// @has - '//*[@class="docblock"]/pre/code' '"unterminated'
+// @has - '//*[@class="docblock"]' '"unterminated'
 /// ```
 /// "unterminated
 /// ```