Do unescaping on Cows instead of slices to fix lifetimes

[stevenroose]

Fixes #329 and relates to #328.

The second commit has lifetime indications that could have been inferred, but it's safer to explicitly add them so that accidentally violating them gets noticed more easily.

Also, is there a way in which I can include my example snippet from #329 as a test somehow? Where would I put that?

[dralley]

Would Into<Cow<..>> work? I ask because requiring Cow<..> for the escape routines would be rather inconvenient

Into<Cow<..>> ought to be implemented for String, &str, and of course Cow<..>

[dralley]

Also, apologies for the inactivity, the original maintainer has been busy for the past few months

[dralley]

This is likely to be addressed soon