When the "unstable" Cargo feature is given, html5ever’s build script
will use html5ever_macros as a library to generate or update
src/tree_builder/rules.expanded.rs, which is part of the source tree.

(This feature also enables the corresponding features of tendril and
string-cache.)

html5ever_macros is not a plugin anymore, but it still uses libsyntax
and still behaves a lot like a syntax extension.

When "unstable" is *not* given, the build script checks that
rules.expanded.rs is up to date (based on a hash of rules.rs) and fails
if it’s not.

owned_dom was an interesting experiment, but there is no use for it.

... even though tokenizer and tree builder test are disabled
as they are created dynamically using internals of the `test` crate.