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/fragment.rs

@@ -886,7 +886,8 @@ impl Fragment {
             SpecificFragmentInfo::InlineBlock(_) => {
                 QuantitiesIncludedInIntrinsicInlineSizes::all()
             }
-            SpecificFragmentInfo::Table | SpecificFragmentInfo::TableCell => {
+            SpecificFragmentInfo::Table |
+            SpecificFragmentInfo::TableCell => {
                 INTRINSIC_INLINE_SIZE_INCLUDES_PADDING |
                     INTRINSIC_INLINE_SIZE_INCLUDES_BORDER |
                     INTRINSIC_INLINE_SIZE_INCLUDES_SPECIFIED