Add slice access

[willglynn]

I added slice access to specs (amethyst/specs#634) to support OpenCL (amethyst/specs#553). Usage looks like:

impl<'a> System<'a> for SomeSystem {
    type SystemData = (WriteStorage<'a, Something>, );

    fn run(&mut self, (mut some_storage, ): Self::SystemData) {
        // 1. get a mutable slice of the component storage
        let slice = some_storage.unprotected_storage_mut().as_mut_slice();

        // 2. process the slice with OpenCL or whatever

        // 3. there is no step 3
    }
}

Components can be processed using an OpenCL kernel:

struct Something {
    float foo;
    float bar;
    float baz;
};

__kernel void process_something(__global struct Something* slice) {
    __global struct Something* something = &slice[get_global_id(0)];
    // do stuff with this data
}

There's a catch when using slices from DefaultVecStorage<T> or VecStorage<T>. The slices are sparse, which means the kernel will either uselessly process T::default() or perform unchecked access into a MaybeUninit<T> for any entity which does not have component T.

Exposing a slice from hibitset::BitSet would allow an OpenCL kernel check the storage mask itself:

struct Something {
    float foo;
    float bar;
    float baz;
};

#define BITS_PER_SIZE_T (sizeof(size_t) * 8)

__kernel void process_something(__global size_t* bitset_layer0, __global struct Something* slice) {
    // check that this index contains data
    size_t mask_index = get_global_id(0) / BITS_PER_SIZE_T;
    size_t shift = get_global_id(0) % BITS_PER_SIZE_T;
    if ((bitset_layer0[mask_index] >> shift) & 1 == 0) {
        return;
    }
  
    __global struct Something* something = &slice[get_global_id(0)];
    // do stuff with this data
}

This PR adds layer{0,1,2}_as_slice(&self) -> &[usize] to BitSet, as well as trivial constants which make correct usage more obvious. Layer 3 is specified to be a single usize so I didn't see any benefit to exposing it as a slice.

[azriel91]

Heya, thank you for this, the docs are really clear ✌️

[azriel91]

bors r+

[bors]

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