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.
The majority of the test modules were already named "tests", and that's
what the example in the Rust book uses, so switch to that for all test
modules, for consistency.
