be generic over the path format and separator

[jordens]

~No real need to enforce a Peekable<Split<char>>. Maybe https://docs.rs/itertools/latest/itertools/trait.PeekingNext.html is enough.~

• [x] get_path() and set_path() are now (with #105) generic over the path format and separator.
• [x] next_path and metadata() need redesign to make them agnostic.

[ryan-summers]

There are a few places where a separator is employed:

1. During derivation to construct setting lookup paths
2. During Miniconf::get() and Miniconf::set() default implementations.
3. In default miniconf::Array and [T; N] implementations (recurse_paths() function)

It seems like the macro could (quite easily) expose a new attribute to specify the separator:

#[derive(Miniconf)]
#[miniconf(separator=':')]
struct Settings {
   // ...
}

During code-gen, we could implement an overriden get() and set() function as well as updated path generation function. I'm not sure how we could fix up the default implementation for array - perhaps the separator could be passed into the recurse_paths function.

[jordens]

Or maybe next_path can be restructured to return an iterator over path parts. Then the user (or iter_paths()) can join each path.