Rollup merge of rust-lang#22869 - alexcrichton:stabilize-env, r=aturon

 Now that the `std::env` module has had some time to bake this commit marks most
of its APIs as `#[stable]`. Some notable APIs that are **not** stable (and still
use the same `env` feature gate) are:

* `{set,get}_exit_status` - there are still questions about whether this is the
  right interface for setting/getting the exit status of a process.
* `page_size` - this may change location in the future or perhaps name as well.

This also effectively closes rust-lang#22122 as the variants of `VarError` are
`#[stable]` now. (this is done intentionally)

Author: Manishearth

src/compiletest/compiletest.rs

@@ -20,7 +20,6 @@
 #![feature(std_misc)]
 #![feature(test)]
 #![feature(unicode)]
-#![feature(env)]
 #![feature(core)]
 
 #![deny(warnings)]

src/liblog/lib.rs

@@ -174,7 +174,6 @@
 #![feature(core)]
 #![feature(old_io)]
 #![feature(std_misc)]
-#![feature(env)]
 
 use std::boxed;
 use std::cell::RefCell;

src/librustc/lib.rs

@@ -31,7 +31,6 @@
 #![feature(int_uint)]
 #![feature(old_io)]
 #![feature(libc)]
-#![feature(env)]
 #![feature(old_path)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]

src/librustc_back/lib.rs

@@ -40,7 +40,6 @@
 #![feature(old_path)]
 #![feature(rustc_private)]
 #![feature(staged_api)]
-#![feature(env)]
 #![feature(path)]
 
 extern crate syntax;

src/librustc_driver/lib.rs

@@ -26,7 +26,6 @@
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]
-#![feature(env)]
 #![feature(int_uint)]
 #![feature(old_io)]
 #![feature(libc)]
@@ -38,6 +37,7 @@
 #![feature(unsafe_destructor)]
 #![feature(staged_api)]
 #![feature(unicode)]
+#![feature(exit_status)]
 
 extern crate arena;
 extern crate flate;

src/librustc_trans/lib.rs

@@ -30,7 +30,6 @@
 #![feature(core)]
 #![feature(int_uint)]
 #![feature(old_io)]
-#![feature(env)]
 #![feature(libc)]
 #![feature(old_path)]
 #![feature(quote)]

src/librustdoc/lib.rs

@@ -22,7 +22,7 @@
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(core)]
-#![feature(env)]
+#![feature(exit_status)]
 #![feature(int_uint)]
 #![feature(old_io)]
 #![feature(libc)]