layout: Rewrite Servo's vertical-align support to match CSS 2.1 §

10.8, and implement `vertical-align: middle` per CSS 2.1 § 10.8.1.

`InlineMetrics` has been split into `InlineMetrics` for fragments and
`LineMetrics` for lines. Both structures' fields have been renamed in
order to more clearly delineate the difference between *space* and
*content*. Vertical positioning of fragments has been reworked to take
margins and borders into account only for replaced content.

This patch fixes the `vertical_align_super_a.html` reftest. Servo now
matches the rendering that Gecko and WebKit produce.

Additionally, this includes a test for the popular inline-block
centering technique described here:
https://s.codepen.io/shshaw/fullpage/gEiDt?#Inline-Block

components/layout/flow.rs

@@ -1576,7 +1576,7 @@ impl ContainingBlockLink {
                 if flow.is_block_like() {
                     flow.as_block().explicit_block_containing_size(shared_context)
                 } else if flow.is_inline_flow() {
-                    Some(flow.as_inline().minimum_block_size_above_baseline)
+                    Some(flow.as_inline().minimum_line_metrics.space_above_baseline)
                 } else {
                     None
                 }