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/test_external/WORKSPACE

@@ -0,0 +1,10 @@
+local_repository(
+    name = "emsdk",
+    path = "..",
+)
+
+load("@emsdk//:deps.bzl", "deps")
+deps()
+
+load("@emsdk//:emscripten_deps.bzl", "emscripten_deps")
+emscripten_deps()