Update `liballoc` to use `KernelError` enum

[Kfeavel]

I think this will require use moving the libs directory from the root to the kernel directory. Not sure how else this will work, especially with SCons.

[micahswitzer]

We can just add the kernel's public header directory to the CPPPATH and #include the header with the enum in it.

Alternatively we could wrap liballoc's API so that it returns a KernelError.

[Kfeavel]

I was thinking about this a bit more and am still erring on the side of moving liballoc into the Kernel directory since the Libraries directory in the root normally implies it's a library that can be used in either kernel or user space. At least, it seems like that's how other projects like Serenity do it.