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

bazel/deps.bzl

@@ -0,0 +1,11 @@
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+def deps():
+    excludes = native.existing_rules().keys()
+
+    if "build_bazel_rules_nodejs" not in excludes:
+        http_archive(
+            name = "build_bazel_rules_nodejs",
+            sha256 = "0f2de53628e848c1691e5729b515022f5a77369c76a09fbe55611e12731c90e3",
+            urls = ["https://github.com/bazelbuild/rules_nodejs/releases/download/2.0.1/rules_nodejs-2.0.1.tar.gz"],
+        )