kernel: use a slab allocator for kmalloc

For a while during development it managed to be faster than the old allocator
despite having more overhead, probably because it makes better use of the
cache. It no longer is - not sure why. The code definitely could be optimized
to hell, though, and it'd probably recover its original edge.

Non-power-of-2 sizeclasses would be really useful, as I could then e.g.
fit four processes into a page instead of three, but I think implementing them
"well" would be too complicated for now.

In hindsight arbitrary allocation descriptions were probably a bad idea too.
I've kept them in for now, but I'll switch to an enum of allocation types soon.
I think I can fit all the per-allocation state (size+type) in 2 bytes.

There are so few allocations in the kernel that I no longer care about the
backtraces, so I'm leaving them out.

The "large" allocations could probably be tested with execbuf in userland, so
I might actually implement them soon too.

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