Methods that used to take `ToCStr` implementors by value, now take them by reference. In particular, this breaks some uses of `Command`:
``` rust
Command::new("foo"); // Still works
Command::new(path) -> Command::new(&path)
cmd.arg(string) -> cmd.arg(&string) or cmd.arg(string.as_slice())
```
[breaking-change]
---
It may be sensible to remove `impl ToCstr for String` since:
- We're getting `impl Deref<str> for String`, so `string.to_cstr()` would still work
- `Command` methods would still be able to use `cmd.arg(string[..])` instead of `cmd.arg(&string)`.
But, I'm leaving that up to the library stabilization process.
r? @aturon
cc #16918