Migrate from Azure Pipelines to Github Actions (#474)

This commit migrates wasmtime's CI infrastructure from Azure Pipelines
to Github Actions. Using Github Actions has a few benefits over other
offerings:

* Being natively integrated with Github means that there's no degree of
  user account configuration or access control management, it's all
  inherent via already existing Github permissions.

* Github Actions gives 20 parallel builders instead of Azure's 10 by
  default, which is a nice boost to have!

Overall I've found Github Actions to feel a bit cleaner than Azure
Pipelines as well. Subjectively I've found the configuration to be more
readable and more pleasant to work with, although they're both just as
"powerful" I think. Additionally Github Actions has been pretty solid in
my own personal testing for a number of other projects.

The main trickiness with wasmtime's CI is the rolling `dev` release of
the master branch as well as binary releases for tags. Github Actions
doesn't have quite as much built in functionality as Azure Pipelines,
but Github Actions does have a nice feature where you can define the
code for an action locally rather than only using built-in actions.

This migration adds three local actions with some associated JS code to
run the action (currently it looks like it basically requires JS)

* An `install-rust` action papers over the gotchas about installing
  Rust, allowing Rust installation to be a one-liner in the configuration.

* A `binary-compatible-builds` action allows easily configuring the
  wheels and the binaries to be "more binary compatible" and handles
  things like compilation flags on OSX and Windows while handling the
  `centos:6` container on Linux.

* The `github-release` action is the logic using the `@actions/github`
  JS package to orchestrate the custom way we manage rolling releases,
  ensuring that a new release is made for the master branch under `dev`
  (deleting the previous tag/release ahead of time) and then also
  manages tagged releases by uploading them there.

I'm hoping that most of the inline actions here will largely go away.
For example `install-rust` should be simply `rustup update $toolchain`
once various environment issues are fixed on Github Actions runner
images. Additionally `github-release` will ideally migrate to something
like https://github.com/actions/create-release or similar once it has
enough functionality. I'm also hoping that the maintenance in the
meantime of these actions is pretty low-cost, but if it becomes an issue
we can look into other solutions!

.github/actions/install-rust/main.js

@@ -0,0 +1,17 @@
+const child_process = require('child_process');
+const toolchain = process.env.INPUT_TOOLCHAIN;
+
+for (var i = 0, keys = Object.keys(process.env), ii = keys.length; i < ii; i++) {
+  console.log(keys[i] + '=' + process.env[keys[i]]);
+}
+
+if (process.platform === 'darwin') {
+  child_process.execSync(`curl https://sh.rustup.rs | sh -s -- -y --default-toolchain=none --profile=minimal`);
+  const bindir = `${process.env.HOME}/.cargo/bin`;
+  console.log(`::add-path::${bindir}`);
+  process.env.PATH = `${process.env.PATH}:${bindir}`;
+  child_process.execFileSync('rustup', ['set', 'profile', 'minimal']);
+}
+
+child_process.execFileSync('rustup', ['update', toolchain, '--no-self-update']);
+child_process.execFileSync('rustup', ['default', toolchain]);