Import and run the upstream spec tests

[tlively]

Now that wasm-shell parses the standard wast format, we should import and run the upstream spec tests, ideally by importing https://github.com/WebAssembly/testsuite as a git submodule. (We already use git submodules to import gtest, so this is not fundamentally different than what we're already doing.)

This will require fixing various bugs in the interpreter and adding support for instructions like table.init that we do not already support.