layout: Float table wrappers directly instead of generating a block
wrapper around them.

Fixes Wikipedia tables leaking out.

Along the way, I refactored tables' width calculation significantly.
This was necessary in order to properly handle floated tables, as some
of the logic had to be ported over from block flows.
Merge pull request #3434 from pcwalton/directly-floated-tables

layout: Float table wrappers directly instead of generating a block

Reviewed-by: glennw