Inline some conversion methods around OsStr
Diff on the assembly of this snippet before and after this PR: https://www.diffchecker.com/NeGMjaJ2
```rust
use std::env;
use std::io;
use std::path::{Path, PathBuf};
pub fn cargo_home_with_cwd(cwd: &Path) -> io::Result<PathBuf> {
match env::var_os("CARGO_HOME").filter(|h| !h.is_empty()) {
Some(home) => {
let home = PathBuf::from(home);
if home.is_absolute() {
Ok(home)
} else {
Ok(cwd.join(&home))
}
}
_ => env::home_dir()
.map(|p| p.join(".cargo"))
.ok_or_else(|| io::Error::new(io::ErrorKind::Other, "could not find cargo home dir")),
}
}
```
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