Update no_std support for Rust 2018 Edition.

With Rust 2018 Edition, the `mod std` trick to alias `core` names to
`std` no longer works, so switch to just having the code use `core`
explicitly.

So instead, switch to just using `core::*` for things that in core.
This is more consistent with other Rust no_std code. And it allows
us to enable `no_std` mode unconditionally in the crates that support
it, which makes testing a little easier.

There actually three cases:

 - For things in std and also in core, like `cmp`: Just use them via
   `core::*`.

 - For things in std and also in alloc, like `Vec`: Import alloc as std, as
   use them from std. This allows them to work on both stable (which
   doesn't provide alloc, but we don't support no_std mode anyway) and
   nightly.

 - For HashMap and similar which are not in core or alloc, import them in
   the top-level lib.rs files from either std or the third-party hashmap_core
   crate, and then have the code use super::hashmap_core.

Also, no_std support continues to be "best effort" at this time and not
something most people need to be testing.

lib/codegen/src/lib.rs

@@ -40,15 +40,20 @@
         clippy::use_self
     )
 )]
-// Turns on no_std and alloc features if std is not available.
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
-// TODO: Remove this workaround once https://github.com/rust-lang/rust/issues/27747 is done.
-#![cfg_attr(not(feature = "std"), feature(slice_concat_ext))]
 
 #[cfg(not(feature = "std"))]
 #[macro_use]
-extern crate alloc;
+extern crate alloc as std;
+#[cfg(feature = "std")]
+#[macro_use]
+extern crate std;
+
+#[cfg(not(feature = "std"))]
+use hashmap_core::{map as hash_map, HashMap, HashSet};
+#[cfg(feature = "std")]
+use std::collections::{hash_map, HashMap, HashSet};
 
 pub use crate::context::Context;
 pub use crate::legalizer::legalize_function;
@@ -101,20 +106,5 @@ mod unreachable_code;
 
 pub use crate::result::{CodegenError, CodegenResult};
 
-/// This replaces `std` in builds with `core`.
-#[cfg(not(feature = "std"))]
-mod std {
-    pub use alloc::{boxed, slice, string, vec};
-    pub use core::*;
-    pub mod collections {
-        #[allow(unused_extern_crates)]
-        extern crate hashmap_core;
-
-        pub use self::hashmap_core::map as hash_map;
-        pub use self::hashmap_core::{HashMap, HashSet};
-        pub use alloc::collections::BTreeSet;
-    }
-}
-
 /// Version number of this crate.
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");