doc: enable OpenType kerning and ligatures

src/doc/rust.css

@@ -62,6 +62,10 @@ body {
     font-size: 18px;
     color: #333;
     line-height: 1.428571429;
+
+    -webkit-font-feature-settings: "kern", "liga";
+    -moz-font-feature-settings: "kern", "liga";
+    font-feature-settings: "kern", "liga";
 }
 @media (min-width: 768px) {
     body {

src/librustdoc/html/static/main.css

@@ -69,6 +69,10 @@ body {
     margin: 0;
     position: relative;
     padding: 10px 15px 20px 15px;
+
+    -webkit-font-feature-settings: "kern", "liga";
+    -moz-font-feature-settings: "kern", "liga";
+    font-feature-settings: "kern", "liga";
 }
 
 h1 {
@@ -101,7 +105,7 @@ h3.impl, h3.method, h4.method {
 h3.impl, h3.method {
     margin-top: 15px;
 }
-h1, h2, h3, h4, section.sidebar, a.source, .search-input, .content table a, .collapse-toggle {
+h1, h2, h3, h4, section.sidebar, a.source, .search-input, .content table :not(code)>a, .collapse-toggle {
     font-family: "Fira Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
 }