Some small changes for consistency

- Methodyfied the string ascii extionsion functions - They got added recently, I wrapped them in a trait.
- Added `into_owned()` method for vectors - similar to `Str`'s `into_owned()` function, allows to convert to a owned vector without making a copy if the source is a owned vector.
- Added `or_some` method to option - similar to `unwrap_or_default`, but keeps the values wrapped in an `Option`. Useful for `Option` chains, eg Iterator impls.
- Added `DoubleEndedIterator` impl to `Option` - Just for compatibility with generic Iterator functions.
- Renamed nil.rs to unit.rs - the type got renamed ages ago, it's time the source file is as well.