Implement tilde expansion for path env

[Xaeroxe]

Fixes #71 by expanding tilde (~) to the value of the home directory. Emulates the bash approach to tilde expansion.

This PR intentionally ignores the more complicated parts of how bash expands the tilde character, because I feel that path resolution using the PATH environment variable should not rely on any information about the current directory, or about the previous current directory. So, ~/dir works, ~+/dir does not.

[Xaeroxe]

Hey @harryfei I'd like to reduce how much I need to bother you for the which-rs repo. To that end I was wondering if you could give me owner access to the crates.io listing for the crate.

If that's not something you're comfortable doing I understand. Maybe you can help me get this PR code reviewed and published in that case?

[harryfei]

Sure, I can. I have send a owner invite to "Xaeroxe" in crates.io.
Feel free to review any PRs, merge them and update which-rs new version.
I'm so busy at my life and work. Thank you for your contribution to this project.

@Xaeroxe