Rename struct slab_alloc to struct slab_allocator.

Rename struct slab_alloc to struct slab_allocator.

We have a function with the following signature:

  void *slab_alloc(struct slab_alloc *);

which produces the following error message with g++:

  error: ‘void* slab_alloc(slab_alloc*)’ hides constructor for ‘struct
  slab_alloc’ [-Werror=shadow]

The two easy ways out are 1) renaming the function or 2) renaming the struct.
For clarity (and consistency with malloc and co) I've chosen to rename the
struct to "struct slab_allocator".

Signed-off-by: Simon Gerber <simon.gerber@inf.ethz.ch>