Move `main`'s arguments out into separate imports.

[sunfishcode]

• Rename main to run to avoid colliding with language toolchains that assign special meaning to the name "main".

• Move run to its own interface.

• Instead of having the run function take stdio, command-line arguments, and preopens as arguments, have separate imports for those. This will make it easier to reuse the run interface in other worlds with other imports.

[sunfishcode]

I've now also split the stdio interface into separate stdin, stdout, and stderr interfaces, to allow worlds to pick which ones they want.

[lukewagner]

Great! This should be imminently useful for wasi-http.