Update selectors to 0.10, with ToCss serialization.
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r? @emilio
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The style candidate cache had regressed a few times (see #12534), and my
intuition is that being able to disable all style sharing with a single rule in
the page is really unfortunate.
This commit redesigns the style sharing cache in order to be a optimistic cache,
but then reject candidates if they match different sibling-affecting selectors
in the page, for example.
So far the numbers have improved, but not so much as I'd wanted (~10%/20% of
non-incremental restyling time in general). The current implementation is really
dumb though (we recompute and re-match a lot of stuff), so we should be able to
optimise it quite a bit.
I have different ideas for improving it (that may or may not work), apart of the
low-hanging fruit like don't re-matching candidates all the time but I have to
measure the real impact.
Also, I need to verify it against try.
* Sections like `[dependencies.foo]` can be entries in a `[dependencies]`
section with the `{key = value}` syntax.
* Per-target dependencies can be expressed with more general `cfg(…)`
conditions instead of exact target triples:
https://github.com/rust-lang/cargo/pull/2328
