dmenv run does not handle non-ascii chars on Windows

[dmerejkowsky]

We use libc::execv directly because it's simple and works on Unix, but it's hard to make it work properly with non-ASCII chars.

Here's the plan:

• Wrap _wexec from the win32 API in Rust
• Use https://doc.rust-lang.org/std/os/windows/ffi/trait.OsStrExt.html#tymethod.encode_wide to convert from Rust strings to wcha_t*
• Call _wexec instead of exec

Note: exec-rs sadly can't be used, because it uses libc::execv as well :/