mp_compile no longer takes an emit_opt argument, rather this setting is now
provided by the global default_emit_opt variable.
Now, when -X emit=native is passed as a command-line option, the emitter
will be set for all compiled modules (included imports), not just the
top-level script.
In the future there could be a way to also set this variable from a script.
Fixes issue #4267.
In this port JavaScript is the underlying "machine" and MicroPython is
transmuted into JavaScript by Emscripten. MicroPython can then run under
Node.js or in the browser.
Damien George
Rami Ali