Refactor assign_height and assign_width for block flow.
Add fixed table test cases
Construct table-related flow and calculate fixed layout
Support a part of anonymous table(step 1-1, 1-2, 2).
Not Covered: step 1-3, 1-4, 3-1, 3-2
*Spec: http://www.w3.org/TR/CSS21/tables.html#anonymous-boxes
auto merge of #1973 : june0cho/servo/table_rebase, r=larsbergstrom,metajack

This is a rebase of #1548 on recent master.

There have been many changes since #1548 is first uploaded, so I'm creating new PR.
This PR includes:
 - construction of table-* flows (table-wrapper, table-caption, table, table-rowgroup, table-row, table-cell)
 - fixed-layout table calculation
 - a part of anonymous table object implementation 
[CSS 2.1, 17.2.1](http://www.w3.org/TR/CSS21/tables.html#anonymous-boxes) (Step 1-1, 1-2, Step 2)