feat: use Yarn workspaces

Much faster installation which is very important for CI.

We'll see if it proves difficult for deps management.

scripts/run

@@ -0,0 +1,20 @@
+#!/usr/bin/env node
+
+const { fromEvent } = require('promise-toolbox')
+const { spawn } = require('child_process')
+
+const { getPackages } = require('./utils')
+
+require('exec-promise')(([ command, ...args ]) =>
+  getPackages().forEach(({ dir, name }) => {
+    console.log('*', name)
+    return fromEvent(spawn(command, args, {
+      cwd: dir,
+      stdio: 'inherit'
+    }), 'exit').then(code => {
+      if (code !== 0) {
+        throw code
+      }
+    })
+  })
+)