* 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
Previously this had to be
emsdk install sdk-releases-upstream-HASH
The only thing preventing using just the hash was that there was no
default for the backend, so defaulting to upstream fixes this. And then
we can do
emsdk install HASH
