std: Move std::env to the new I/O APIs

This commit moves `std::env` away from the `std::old_io` error type as well as
the `std::old_path` module. Methods returning an error now return `io::Error`
and methods consuming or returning paths use `std::path` instead of
`std::old_path`. This commit does not yet mark these APIs as `#[stable]`.

This commit also migrates `std::old_io::TempDir` to `std::fs::TempDir` with
essentially the exact same API. This type was added to interoperate with the new
path API and has its own `tempdir` feature.

Finally, this commit reverts the deprecation of `std::os` APIs returning the old
path API types. This deprecation can come back once the entire `std::old_path`
module is deprecated.

[breaking-change]

src/libstd/dynamic_lib.rs

@@ -21,6 +21,7 @@ use ffi::CString;
 use mem;
 use env;
 use str;
+use os;
 
 pub struct DynamicLibrary {
     handle: *mut u8
@@ -102,7 +103,7 @@ impl DynamicLibrary {
     /// process
     pub fn search_path() -> Vec<Path> {
         match env::var_os(DynamicLibrary::envvar()) {
-            Some(var) => env::split_paths(&var).collect(),
+            Some(var) => os::split_paths(var.to_str().unwrap()),
             None => Vec::new(),
         }
     }