Add missing clean-* make targets.

Some more recent Servo crates were not cleaned by `make servo-clean`.
Add tests for the bit_struct! macro.

The macro crate, when built with `--test`,
now depends on itself (a version of itself built without `--test`).
This is a bit unusal, but allows testing a syntax extension.
I did not feel like making a new crate just for this,
and these tests do not belong in any of the other existing crates.
Remove obsolete TODO comments
Only write the px value in Show for Au

The integer Au value is mostly internal and does not make much sense.

Printing both values is very verbose and hurts readability of debug logs.
Dump display lists when debug!() is enabled for gfx::display_list
Convert back to physical some layout coordinates.
Set the origin of the root element per its writing mode.

It’s not always (0, 0) anymore.
Reftests can opt into --experimental
Add a reftest for basic block layout in a vertical writing mode.
Merge pull request #3152 from SimonSapin/clean

Add missing clean-* make targets.
Merge pull request #3160 from mbrubeck/old-layout-comments

Remove obsolete TODO comments
Merge pull request #3110 from SimonSapin/writing-modes-reftest

Add a basic CSS Writing Modes reftest
Remove a .orig file left over from conflict resolution.
Merge pull request #3153 from SimonSapin/bit-struct-tests

Add tests for the bit_struct! macro. r=mbrubeck
Enable some of the HTML tests.
Split up web-platform-tests on Travis.