Move PropertyBitField to a syntax extension.

This is a first step towards moving away the Mako template in the style crate.
fixup! Move PropertyBitField to a syntax extension.
Merge pull request #3142 from SimonSapin/bit-struct-macro

Move PropertyBitField to a syntax extension.
Fixed AttrValue::from_tokenlist indexes

The following issues were found:
 - Single concatenated spaces were indexed as a single token;
 - Last token, if not followed by an HTML space character, was ignored;
Implemented Attribute's tokens() iterator
Use AttrValue::tokens() in Element::has_class()
Implement DOMTokenList.contains
Cleaned DOMTokenList code duplication
Mark WPT tests as FAIL due to lack of DOMTokenList.toggle

For now, these tests are being handled in test_element_classList.html
until DOMTokenList.toggle gets implemented.

Created issue #3138 to track DOMTokenList.toggle implementation.
Merge pull request #3061 from brunoabinader/domtokenlist

Implement DOMTokenList.contains; r=Ms2ger
Add https support
Merge pull request #3148 from glennw/ssl

Add https support
Add documentation about web-platform-tests.
Merge pull request #3150 from Ms2ger/wpt-docs

Add documentation about web-platform-tests; r=jgraham
Load bare images within an HTML document (#3108)
Update ProxyTraps to use real bools.
Merge pull request #3151 from Ms2ger/bools

Update ProxyTraps to use real bools; r=jdm
Merge pull request #3131 from Manishearth/image

Load bare images within an HTML document (#3108); r=Ms2ger
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.