We use the [`alloc`](https://doc.rust-lang.org/alloc/) create, which requires the user define a global allocator.

* Import from `core` and `alloc` instead of `std`
* Create `std` feature
* Create `use-bare-io` feature which adds a
  [bare-io](https://github.com/bbqsrc/bare-io) dependency to polyfill
  `std::io` features. This is an experimental feature and should be
  used with extreme caution.
* Created an `io` module to polyfill `std::io` with `bare-io` in
  `no_std`.
* Replace `HashMap` and `HashSet` with `BTreeMap` and `BTreeSet` just to
  get something working. This needs to be fixed.
* CI runs tests `no_std` code

`std::collections::HashMap` and `std::collections::HashSet` don't have
any analogue in `alloc` because they require a OS RNG.

[Recently no_std-compatible implementations have been
merged](rust-lang/rust#58623) but I think we'd
need to massively increase our MSRV.

So I opted to move any `HasMap` or `HasSet` usage in tests to `BTreeMap` or `BTreeSet`, and to gate the only non-test usage of `HashMap` or `HashSet` (in the merkle block code which won't be helpful on MCUs) behind a "std" feature flag. Public API doesn't change.

Upgraded bare-io to a branch which adds a feature to disable the
`#[non_exhaustive]` attribute, which wasn't stabilized until 1.40.0.