implement length for maps

src/builtin/list.rs

@@ -14,6 +14,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             max_args!(args, 1);
             let len = match arg!(args, 0, "list") {
                 Value::List(v, ..) => Number::from(v.len()),
+                Value::Map(m) => Number::from(m.len()),
                 _ => Number::one(),
             };
             Ok(Value::Dimension(len, Unit::None))

src/value/map.rs

@@ -20,6 +20,10 @@ impl SassMap {
         Ok(None)
     }
 
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
     #[allow(dead_code)]
     pub fn remove(&mut self, key: &Value) {
         self.0.retain(|(ref k, ..)| k != key);

tests/map.rs

@@ -76,3 +76,8 @@ test!(
     "a {\n  color: inspect((a: b, c: d, e: f, g: h, i: j, h: k, l: m, n: o));\n}\n",
     "a {\n  color: (a: b, c: d, e: f, g: h, i: j, h: k, l: m, n: o);\n}\n"
 );
+test!(
+    map_length,
+    "a {\n  color: length((a: b, c: d, e: f));\n}\n",
+    "a {\n  color: 3;\n}\n"
+);