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.
This used to conflict with the util crate from the standard library, which
has long since been removed.
The import in layout has not been changed because of a conflict with the
util mod there.
