Build runtime/stdlib files with rescript/bsb instead of ninja.js

[cknitt]

This removes the infamous ninja.js and instead builds all the runtime/stdlib files using the standard rescript/bsb tooling.

For this to work, I had to

• tweak the behavior of the existing -make-runtime option a little
• put all runtime/stdlib files in a single, flat ReScript project (subdirectory runtime)

This is the only setup in which I got it to work. But I think that's good enough because

1. We will eventually move Belt and Js out to separate packages anyway. So in the end only the actual "runtime" files will remain under runtime.
2. Next step will be to get it to work with rewatch instead of bsb which may also allow for some further restructuring.

All tests are passing, there are no changes to the JS output or the artifact list, and the resulting npm package seems to work fine.

[zth]

To me this looks good, but I'm less familiar with this part, so interested in what @cometkim says.