Improve `Path` ergonomics

[Kinrany]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Path is central to object_store, but manipulating values of this type can be hard because its API is mostly structured around one-way parsing.

Describe the solution you'd like
Expand Paths API. In no particular order:

• fn empty() constructor, equivalent to Path::from("/")
• fn is_empty(&self) -> bool, true if self.raw is empty
• fn len(&self) -> usize, equivalent to .parts().count()
• fn prefix(&self) -> Self, dual to the existing Parts::filename
  • Bikeshedding: fn folder?
• Change parts to return a custom iterator that is fused and double-ended.
• Implement Expand in addition to FromIterator

Describe alternatives you've considered
One could argue that Path is not designed to be used as a standalone type. But then I would expect it to be internal to object_store, with all APIs using String instead.

Additional context
PR: #546

[Kinrany]

One more: a conversion of Path into std::path::PathBuf. Which cannot currently be implemented without unnecessary cloning with the current fn parts design.

[kylebarron]

I think Path exists because there are multiple distinct ways to construct a path: Path::from and Path::parse do different things.

[Kinrany]

I wonder if replacing Path with String plus a pathpart escaping function would be enough.

[tustvold]

I don't think we can replace the path abstraction with String or std::path::Path as the Path abstraction handles a number of things that are important for the crate to work correctly. In no particular order:

• It prevents paths with leading or trailing /
• It prevents empty or relative (..) path segments
• It disallows control characters

These are all fundamental to how the API operates, and allows it to map to stores with varying characteristics, especially w.r.t directories.

For more information see https://docs.rs/object_store/latest/object_store/path/struct.Path.html

That being said ergonomic improvements to Path are welcome