Enable no_std use via the addition of a default-enabled feature flag.

[nuttycom]

This allows users who specify nonempty as a --no-default-features dependency can use the crate in environments where alloc is available but std is not, such as embedded device firmware.

This requires edition = 2021, which implies a change to an MSRV of 1.56, therefor , will require a semver bump for release.

[FintanH]

It looks good to me 👌 @cloudhead any chance we could get this in a cut a release for @nuttycom? 🙏

[FintanH]

@nuttycom just updated to see if the build looks fine with the clippy changes

[FintanH]

@nuttycom could we add these checks to the workflow:

  build:
    name: no-std build  and test
    strategy:
      matrix:
        os: ['ubuntu-latest', 'macos-latest']
    runs-on: ${{ matrix.os }}
    steps:
      - uses: actions/checkout@v2
      - name: Build
        run: cargo build --verbose --no-default-features
        env:
          RUSTFLAGS: -D warnings
      - name: Run tests
        run: cargo test --all --verbose --no-default-features

Because otherwise, I don't think our checks will ensure that no-std is working

[FintanH]

Actually, I have it in #65 :)

[FintanH]

I've merged #65 so I'll close this.

I'll cut a release with a semver bump now :)