* Optimize sandbox performance
Link just the files needed to compile, link, or archive, rather than the entire directory tree. This drastically improves build times on macOS, where the sandbox is known to be slow (https://github.com/bazelbuild/bazel/issues/8230).
* Linux wants clang to link
* all_files not needed?
* Linux wants llc
* And llvm-ar
* Templated build_file_content
* 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
