Use emsdk as external bazel dependency (#766)

* Makes provided bazel rules look up @emsdk workspace instead of local workspace
* Uses system-specific emscripten binaries instead of defaulting to linux
* Provides macros for loading emsdk dependencies (nodejs and emscripten binaries)
* Unhardcodes paths in bazel rules and .sh wrappers
* `update_bazel_workspace.sh` now updates `revisions.bzl`
* `emscripten_deps()` can be fed with specific emscripten version
* Adds external usage test

Addresses #650 and #696

.circleci/config.yml

@@ -166,7 +166,7 @@ jobs:
             docker login -u "$DOCKER_USER" -p "$DOCKER_PASS"
             make -C ./docker version=${CIRCLE_TAG} alias=latest push
 
-  test-bazel:
+  test-bazel-linux:
     executor: bionic
     steps:
       - checkout
@@ -186,6 +186,21 @@ jobs:
             apt-get install -q -y bazel
       - run: test/test_bazel.sh
 
+  test-bazel-mac:
+    macos:
+      xcode: "12.2.0"
+    environment:
+      EMSDK_NOTTY: "1"
+      HOMEBREW_NO_AUTO_UPDATE: "1"
+    steps:
+      - checkout
+      - run: brew install grep
+      - run:
+          name: install bazel
+          command: |
+            brew install bazel
+      - run: test/test_bazel_mac.sh
+
 workflows:
   flake8:
     jobs:
@@ -208,6 +223,9 @@ workflows:
               ignore: /.*/
             tags:
               only: /.*/
-  test-bazel:
+  test-bazel-linux:
     jobs:
-      - test-bazel
+      - test-bazel-linux
+  test-bazel-mac:
+    jobs:
+      - test-bazel-mac