Source files are copied to the out directory

1. `make` copies the source files to `out`
2. Go-to-definition in the `test/` directory leads to `out`
3. I accidentally modify files in `out`
4. `make` silently reverts my changes on the next build

This has bitten me so many times now. I'm really tired of the `out` business. We need to fix this.
- Could we symlink instead of copy?
- Or at least make the files read-only?
- Or do away with out entirely?

A related issue is that we have two copies of `pulse.cmxs`, and `make plugin` only updates one of them.