multiple build processes, processes disagree

[sunfishcode]

When using src/Makefile, one gets an executable at src/wasm. However, runtests.py expects an executable at src/main.native. It'd be useful for these to agree; which one is preferable?

[kg]

Happy to update runtests to handle this. I personally dislike what ocamlbuild does.

[rossberg]

SGTM as well. Would be nicer if we did get a binary named wasm in a consistent place (preferably outside the src dir). Can one do that with ocamlbuild?

[sunfishcode]

It looks like #83 would fix this.