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/preopt/src/constant_folding.rs

@@ -102,7 +102,7 @@ fn resolve_value_to_imm(dfg: &ir::DataFlowGraph, value: ir::Value) -> Option<Con
 }
 
 fn evaluate_binary(opcode: ir::Opcode, imm0: ConstImm, imm1: ConstImm) -> Option<ConstImm> {
-    use std::num::Wrapping;
+    use core::num::Wrapping;
 
     match opcode {
         ir::Opcode::Iadd => {

lib/preopt/src/lib.rs

@@ -18,11 +18,15 @@
         clippy::use_self
     )
 )]
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
 #![cfg_attr(not(feature = "std"), feature(alloc))]
 
 #[cfg(not(feature = "std"))]
-extern crate alloc;
+#[macro_use]
+extern crate alloc as std;
+#[cfg(feature = "std")]
+#[macro_use]
+extern crate std;
 
 mod constant_folding;
 
@@ -49,10 +53,3 @@ where
     ctx.verify_if(fisa)?;
     Ok(())
 }
-
-/// This replaces `std` in builds with `core`.
-#[cfg(not(feature = "std"))]
-mod std {
-    pub use alloc::{boxed, slice, string, vec};
-    pub use core::*;
-}