layout: Implement border-spacing per CSS 2.1 § 17.6.1 and the legacy

`cellspacing` attribute per HTML5 § 14.3.9.

Table layout code has been refactored to push the spacing down to
rowgroups and rows; this will aid the implementation of
`border-collapse` as well.

This commit also fixes two nasty issues in table layout:

* In fixed layout, extra space would not be divided among columns that
  had auto width but had nonzero minimum width.

* In automatic layout, extra space would be distributed to constrained
  columns as well even if unconstrained columns with percentage equal to
  zero were present.

components/layout/wrapper.rs

@@ -635,8 +635,8 @@ impl<'le> TElement<'le> for LayoutElement<'le> {
     #[inline]
     fn has_nonzero_border(self) -> bool {
         unsafe {
-            match self.element
-                      .get_unsigned_integer_attribute_for_layout(UnsignedIntegerAttribute::Border) {
+            match self.element.get_unsigned_integer_attribute_for_layout(
+                    UnsignedIntegerAttribute::Border) {
                 None | Some(0) => false,
                 _ => true,
             }