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/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();