Add std::process

Per [RFC 579](https://github.com/rust-lang/rfcs/pull/579), this commit
adds a new `std::process` module. This module is largely based on the
existing `std::old_io::process` module, but refactors the API to use
`OsStr` and other new standards set out by IO reform.

The existing module is not yet deprecated, to allow for the new API to
get a bit of testing before a mass migration to it.

src/libstd/path.rs

@@ -942,6 +942,11 @@ impl PathBuf {
 
         true
     }
+
+    /// Consume the `PathBuf`, yielding its internal `OsString` storage
+    pub fn into_os_string(self) -> OsString {
+        self.inner
+    }
 }
 
 impl<'a, P: ?Sized + 'a> iter::FromIterator<&'a P> for PathBuf where P: AsPath {