There existed issues related to multithreaded tests that are difficult
to resolve. In the future interning may be reimplemented but in a more
limited capacity.
The motivation behind interning *values* was that it appeared checking
for named colors was responsible for much of the time lost when parsing
unquoted strings. If interning were to be reimplemented, it may make
sense to limit it solely to identifiers and style properties.
Before this commit, escape sequences above std::char::MAX ('\u{10ffff}')
would overflow and cause a panic. This commit replaces an `unwrap` with
`ok_or` and a clearer error message. This message will likely change
in the future in order to better conform to the `dart-sass` implementation
which currently also fails to cleanly handle this overflow.
See https://github.com/kaj/rsass/pull/73
