Verify crates are publish-able on CI (#2036)

This commit updates our CI to verify that all crates are publish-able at
all times on every commit. During the 0.19.0 release we found another
case where the crates as they live in this repository weren't
publish-able, so the hope is that this no longer comes up again!

The script added in this commit also takes the time/liberty to remove
the existing bump/publish scripts and instead replace them with one Rust
script originally sourced from wasm-bindgen. The intention of this
script is that it has three modes:

* `./publish bump` - bumps version numbers which are sent as a PR to get
  reviewed (probably with a changelog as well)

* `./publish verify` - run on CI on every commit, builds every crate we
  publish as if it's being published to crates.io, notably without raw
  access to other crates in the repository.

* `./publish publish` - publishes all crates to crates.io, passing the
  `--no-verify` flag to make this a much speedier process than it is
  today.

.github/workflows/main.yml

@@ -558,3 +558,19 @@ jobs:
     - run: |
         cargo install --root ${{ runner.tool_cache }}/cargo-audit --version ${{ env.CARGO_AUDIT_VERSION }} cargo-audit
         cargo audit
+
+  verify-publish:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: true
+    - run: rustup update stable && rustup default stable
+    - run: |
+        cd ${{ runner.tool_cache }}
+        curl -L https://github.com/mozilla/sccache/releases/download/0.2.13/sccache-0.2.13-x86_64-unknown-linux-musl.tar.gz | tar xzf -
+        echo "::add-path::`pwd`/sccache-0.2.13-x86_64-unknown-linux-musl"
+        echo ::set-env name=RUSTC_WRAPPER::sccache
+    - run: |
+        rustc scripts/publish.rs
+        ./publish verify