Add unicode escaping in resolver

[zbraniecki]

Fixes #79

[stasm]

LGTM although I think I'd prefer unescape_unicode to always return a new String rather than Option<String>.

[zbraniecki]

@bjorn3 - I updated the code to use Cow. I kept the Option<String> internally to determine if the owned string has been initialized, but I return the Cow now. The performance remains the same while the ergonomics improved!

Do you want to take a look at the result to verify that it works as you imagined?