This should make it somewhat easier to experiment with alternative
representations in the future. To reduce churn, this commit leaves the String
field public, though.
Also, this will allow us to use the default String type to represent the IDL
USVString type, which explicitly forbids unpaired surrogates, ans as such is
a better match to the Rust String type.
Extracted out of #5649
* add more hyperlinks to associated specification for structs/methods
* follow redirects and update links
* replace broken links
* removal of WHATWG multipage page name since the page name is not
guaranteed to be stable
For the majority of these cases, `as_slice` can be removed due to
`Deref`. In particular, `Deref` for:
* `String` -> `str`
* `Atom` -> `str`
The latter of those two requires, a bump of the locked `string-cache`
library
