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/bforest/Cargo.toml

@@ -17,6 +17,7 @@ cranelift-entity = { path = "../entity", version = "0.26.0", default-features =
 [features]
 default = ["std"]
 std = ["cranelift-entity/std"]
+core = []
 
 [badges]
 maintenance = { status = "experimental" }

lib/bforest/src/lib.rs

@@ -31,24 +31,24 @@
         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))]
 
-/// This replaces `std` in builds with `core`.
+#[cfg(test)]
 #[cfg(not(feature = "std"))]
-mod std {
-    extern crate alloc;
-    pub use self::alloc::{boxed, string, vec};
-    pub use core::*;
-}
+#[macro_use]
+extern crate alloc as std;
+#[cfg(test)]
+#[cfg(feature = "std")]
+#[macro_use]
+extern crate std;
 
 #[macro_use]
 extern crate cranelift_entity as entity;
 use crate::entity::packed_option;
 
-use std::borrow::BorrowMut;
-use std::cmp::Ordering;
+use core::borrow::BorrowMut;
+use core::cmp::Ordering;
 
 mod map;
 mod node;

lib/bforest/src/map.rs

@@ -3,8 +3,8 @@
 use super::{Comparator, Forest, Node, NodeData, NodePool, Path, INNER_SIZE};
 use crate::packed_option::PackedOption;
 #[cfg(test)]
-use std::fmt;
-use std::marker::PhantomData;
+use core::fmt;
+use core::marker::PhantomData;
 #[cfg(test)]
 use std::string::String;
 
@@ -429,7 +429,7 @@ where
 mod tests {
     use super::super::NodeData;
     use super::*;
-    use std::mem;
+    use core::mem;
     use std::vec::Vec;
 
     #[test]

lib/bforest/src/node.rs

@@ -1,8 +1,8 @@
 //! B+-tree nodes.
 
 use super::{slice_insert, slice_shift, Forest, Node, SetValue, INNER_SIZE};
-use std::borrow::{Borrow, BorrowMut};
-use std::fmt;
+use core::borrow::{Borrow, BorrowMut};
+use core::fmt;
 
 /// B+-tree node.
 ///
@@ -584,7 +584,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::mem;
+    use core::mem;
     use std::string::ToString;
 
     // Forest impl for a set implementation.

lib/bforest/src/path.rs

@@ -2,11 +2,11 @@
 
 use super::node::Removed;
 use super::{slice_insert, slice_shift, Comparator, Forest, Node, NodeData, NodePool, MAX_PATH};
-use std::borrow::Borrow;
-use std::marker::PhantomData;
+use core::borrow::Borrow;
+use core::marker::PhantomData;
 
 #[cfg(test)]
-use std::fmt;
+use core::fmt;
 
 pub(super) struct Path<F: Forest> {
     /// Number of path entries including the root and leaf nodes.
@@ -706,7 +706,7 @@ impl<F: Forest> fmt::Display for Path<F> {
 mod tests {
     use super::super::{Forest, NodeData, NodePool};
     use super::*;
-    use std::cmp::Ordering;
+    use core::cmp::Ordering;
 
     struct TC();
 

lib/bforest/src/pool.rs

@@ -5,8 +5,8 @@ use super::Comparator;
 use super::{Forest, Node, NodeData};
 use crate::entity::PrimaryMap;
 #[cfg(test)]
-use std::fmt;
-use std::ops::{Index, IndexMut};
+use core::fmt;
+use core::ops::{Index, IndexMut};
 
 /// A pool of nodes, including a free list.
 pub(super) struct NodePool<F: Forest> {
@@ -84,8 +84,8 @@ impl<F: Forest> NodePool<F> {
         F::Key: fmt::Display,
     {
         use crate::entity::SparseSet;
-        use std::borrow::Borrow;
-        use std::cmp::Ordering;
+        use core::borrow::Borrow;
+        use core::cmp::Ordering;
         use std::vec::Vec;
 
         // The root node can't be an inner node with just a single sub-tree. It should have been

lib/bforest/src/set.rs

@@ -3,8 +3,8 @@
 use super::{Comparator, Forest, Node, NodeData, NodePool, Path, SetValue, INNER_SIZE};
 use crate::packed_option::PackedOption;
 #[cfg(test)]
-use std::fmt;
-use std::marker::PhantomData;
+use core::fmt;
+use core::marker::PhantomData;
 #[cfg(test)]
 use std::string::String;
 
@@ -357,7 +357,7 @@ where
 mod tests {
     use super::super::NodeData;
     use super::*;
-    use std::mem;
+    use core::mem;
     use std::vec::Vec;
 
     #[test]