The decode logic should be done on Cows instead of slices

[stevenroose]

Just like I did #330 for unescaping, the same should be done for decoding. The Cow's that are currently returned make very little sense, because they usually borrow into ephemeral data so they will always eventually have to be converted into owned data. I'm thinking of having some kind of Bytes-based implementation that might allow us to not have to re-allocate so often when we already have a Cow::Owned.

[dralley]

#158 will address this