ToHtml trait for docs and some rudimentary syntax highlighting that is definitely a work in progress

docs/src/def.rs

@@ -0,0 +1,57 @@
+use crate::html::ToHtml;
+use roc_parse::ast::Def;
+
+impl<'a> ToHtml<'a> for Def<'a> {
+    fn css_class(&self) -> Option<&'a str> {
+        match self {
+            // Def::Annotation(_, _) => {}
+            // Def::Alias { .. } => {}
+            Def::Body(_, _) => None,
+            // Def::AnnotatedBody { .. } => {}
+            // Def::Expect(_) => {}
+            Def::SpaceBefore(_, _) => None,
+            Def::SpaceAfter(_, _) => None,
+            // Def::NotYetImplemented(_) => {}
+            _ => None,
+        }
+    }
+
+    fn html_body(&self, buf: &mut bumpalo::collections::String<'a>) {
+        match self {
+            // Def::Annotation(_, _) => {}
+            // Def::Alias { .. } => {}
+            Def::Body(pattern, expr) => {
+                pattern.html(buf);
+                EqualSign.html(buf);
+                expr.html(buf);
+            }
+            // Def::AnnotatedBody { .. } => {}
+            // Def::Expect(_) => {}
+            Def::SpaceBefore(sub_def, spaces) => {
+                for space in spaces.iter() {
+                    space.html(buf);
+                }
+                sub_def.html(buf);
+            }
+            Def::SpaceAfter(sub_def, spaces) => {
+                sub_def.html(buf);
+                for space in spaces.iter() {
+                    space.html(buf);
+                }
+            }
+            // Def::NotYetImplemented(_) => {}
+            _ => {}
+        }
+    }
+}
+
+struct EqualSign;
+
+impl<'a> ToHtml<'a> for EqualSign {
+    fn css_class(&self) -> Option<&'a str> {
+        Some("operator")
+    }
+    fn html_body(&self, buf: &mut bumpalo::collections::String<'a>) {
+        buf.push_str(" = ")
+    }
+}