Add typed-path versions for current_exe and temp_dir, including UTF8 variants

[simongoricar]

Hello! I'm currently migrating to typed-path in my project and noticed that there were two path-related functions from std::env that I could not find in the utilities module.

This pull request adds the following functions under typed_path::utils:

• current_exe, which corresponds to std::env::current_exe,
• utf8_current_exe, which is a UTF8 version of current_exe,
• temp_dir, which corresponds to std::env::temp_dir, and
• utf8_temp_dir, which is a UTF8 version of temp_dir.

[chipsenkbeil]

Awesome! Looks like there's some linter failures related to doc comments. On my phone, so can't quite tell what the reason is right now.

[simongoricar]

Seems unrelated - looks like it doesn't like the fn main in README.md doctests (the failure is error: needless fn main in doctest six times).

[chipsenkbeil]

Ah, I guess you're right! Most likely, this is some addition to the linter that happened since the last commit to the project. Let's merge this and I'll take a look at correcting the rest.

[chipsenkbeil]

@simongoricar published as 0.9.0. Thanks for the contribution!

[simongoricar]

A pleasure, thanks for the quick release!