Return `String` from `StringLit<String>::into_value`

[Lukas Kalbertodt]

Currently Cow<'static, str> is returned, but it's always Cow::Owned. I wonder whether its more useful to have a simpler type (String instead of Cow<'static, str>) in the common case or whether always getting a Cow back is more useful. The same goes for ByteStringLit::into_value.