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/legalizer/split.rs

@@ -67,7 +67,7 @@
 use crate::cursor::{Cursor, CursorPosition, FuncCursor};
 use crate::flowgraph::{BasicBlock, ControlFlowGraph};
 use crate::ir::{self, Ebb, Inst, InstBuilder, InstructionData, Opcode, Type, Value, ValueDef};
-use std::iter;
+use core::iter;
 use std::vec::Vec;
 
 /// Split `value` into two values using the `isplit` semantics. Do this by reusing existing values