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/extname.rs

@@ -5,9 +5,9 @@
 //! Cranelift, which compiles functions independently.
 
 use crate::ir::LibCall;
-use std::cmp;
-use std::fmt::{self, Write};
-use std::str::FromStr;
+use core::cmp;
+use core::fmt::{self, Write};
+use core::str::FromStr;
 
 const TESTCASE_NAME_LENGTH: usize = 16;
 
@@ -120,8 +120,8 @@ impl FromStr for ExternalName {
 mod tests {
     use super::ExternalName;
     use crate::ir::LibCall;
+    use core::u32;
     use std::string::ToString;
-    use std::u32;
 
     #[test]
     fn display_testcase() {