Make path::Expression module public

[hkr1990]

I'm working on feature control library and would like to take dependency on Value and path::Expression. Unfortunately, path::Expression is not public module. It is useful even outside the context of config-rs. Can you make it public in the next release?

[matthiasbeyer]

Hi! Can you show a usecase why you would need these?

[polarathene]

Closing as no response.