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.

.gitignore

@@ -16,3 +16,6 @@ tags
 target
 .z3-trace
 foo
+.cargo
+publish
+vendor