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/ir/progpoint.rs

@@ -2,9 +2,9 @@
 
 use crate::entity::EntityRef;
 use crate::ir::{Ebb, Inst, ValueDef};
-use std::cmp;
-use std::fmt;
-use std::u32;
+use core::cmp;
+use core::fmt;
+use core::u32;
 
 /// A `ProgramPoint` represents a position in a function where the live range of an SSA value can
 /// begin or end. It can be either: