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.
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