change x= to *=, add unit tests and improve existing tests

spec/data/layouts/extends-units-mixed.yml

@@ -1,8 +1,12 @@
 parent:
-  x: 0.5in
+  x: 100
   y: 1in
+  width: 200
+  height: 1cm
 
 child:
   extends: parent
   x: += 1in
-  y: -= 0.5in
+  y: -= 1cm
+  width: "*= 1.5"
+  height: /= 4

spec/layout_parser_spec.rb

@@ -206,8 +206,13 @@ describe Squib::LayoutParser do
   it 'does mixed unit conversion when extending' do
     layout = subject.load_layout(layout_file('extends-units-mixed.yml'))
     expect(layout).to eq({
-      'parent' => { 'x' => '0.5in', 'y' => '1in'},
-      'child'  => { 'x' => 450.0, 'y' => 150.0, 'extends' => 'parent' },
+      'parent' => {
+        'x' => 100, 'y' => '1in',
+        'width' => 200, 'height' => '1cm' },
+      'child'  => {
+        'x' => 400.0, 'y' => 181.8897639,
+        'width' => 300.0, 'height' => 29.527559025,
+        'extends' => 'parent' },
     })
   end