Update syn to 0.11

[nox]

On crates.io:

• Make match_ignore_ascii_case more efficient, add ascii_case_insensitive_phf_map rust-cssparser#122
• Update syn to 0.11 heapsize#75
• Update syn to 0.11 html5ever#255
• Update syn to 0.11 mystor/synstructure#3

In this repo:

• deny_public_fields
• domobject_derive
• jstraceable_derive

Other:

• Replace dwrote’s dependency on serde_codegen by serde_derive
• Replace webrender’s dependency on serde_codegen by serde_derive

[SimonSapin]

I edited @nox’s message above to reflect the current state with #15588.

[nox]

Thanks @SimonSapin!

[nox]

I removed the bullets for removing serde_codegen from the WR stack, given the current issues to manage dwrote.

[nox]

That is done.