fix load_all()

I think there is a mistake line 37 of `compile-dll.r`: `"inst"` must be `inst`.
Otherwise `file.copy` quietly fails.
I also suggest to look at the returned boolean and throw an error.
If I remember correctly, this will only work correctly if the package directory is the same as the name of the package. That assumption is true for installed packages, but not necessarily for in-development packages.

The general problem is that the directory structure of installed packages isn't the same as that for source code, and I don't know of a good way around that.