Packed Array?

I'm working on a packed array type, where multiple values are packed in a single byte like `BitArray`, but each element can be two or four bits. Each element is considered a `UInt8` value.

```julia
const _msk8 = ~UInt8(0)
@inline _msk_end(l::Int) = _msk8 >>> _mod8(-l)
@inline _div(l, lgbits_per_entry) = l >> (3 - lgbits_per_entry)
@inline _div8(l) = l >> 3
@inline _mod(l, lgbits_per_entry) = l & _msk_end(3 - lgbits_per_entry)
@inline _mod8(l) = l & 0x07
@inline _blsr(x)= x & (x-1) #zeros the last set bit. Has native instruction on many archs. needed in multidimensional.jl
@inline num_chunks(n::Int, lgbits_per_entry::Int) = _div8((n << lgbits_per_entry) + 7)
mutable struct BitsArray{N, B} <: AbstractArray{UInt8, N}
chunks::Array{UInt8,N}
len::Int
dims::NTuple{N,Int}
end

# B: log2(bits per entry)
function BitsArray{N, B}(::UndefInitializer, dims::Vararg{Int,N}) where {N, B}
@assert B ∈ (0, 1, 2)
n = 1
i = 1
nc = 1
dim1 = 0
length(dims) >= 1 && (dim1 = num_chunks(dims[1], B))
for d in dims
d >= 0 || throw(ArgumentError("dimension size must be ≥ 0, got $d for dimension $i"))
n *= d
if i == 1
nc *= dim1
else
nc *= d
end
i += 1
end
length(dims) == 0 && (nc = 0)

chunks = Array{UInt8,N}(undef, dim1, dims[2:end]...)
#nc > 0 && (chunks[end] .= UInt8(0))
b = BitsArray{N, B}(chunks, n, dims)
N != 1 && (b.dims = dims)
return b
end

@inline Base.size(x::BitsArray) = x.dims
@inline Base.length(x::BitsArray) = x.len
Base.IndexStyle(::Type{<:BitsArray{N,B}}) where {N, B} = IndexCartesian()
@inline get_chunks_id(i::Int, lgbits_per_entry::Int) = _div(i-1, lgbits_per_entry) + 1, _mod(i-1, lgbits_per_entry)
# note: within-byte index is meant to be 0-based.

function Base.getindex(a::BitsArray{N,B}, I::Vararg{Int, N}) where {N,B}
@boundscheck checkbounds(a, I...)
@inbounds i1, i2 = get_chunks_id(I[1], B)
bits_per_array = 1 << B
u = _msk_end(bits_per_array) << (i2 * (bits_per_array))
@inbounds r = (a.chunks[i1, I[2:end]...] & u) >> (i2 * bits_per_array)
end
```
The first dimension is to be byte-aligned.

Is there a way for this to work with LoopVectorization? I tried implementing some of the `ArrayInterface.jl` methods, but I must be missing something. Maybe it is more complicated with custom getindex function?

```julia
using ArrayInterface
ArrayInterface.defines_strides(::Type{<:BitsArray}) = true
ArrayInterface.contiguous_axis(::Type{<:BitsArray}) = ArrayInterface.One()
ArrayInterface.stride_rank(::Type{BitsArray{N,B}}) where {N,B} = ArrayInterface.nstatic(Val(N))
ArrayInterface.contiguous_batch_size(::Type{BitsArray{N,B}}) where {N,B} = ArrayInterface.Zero()
ArrayInterface.dense_dims(::Type{BitsArray{N,B}}) where {N,B} = ArrayInterface._all_dense(Val{N}())

ArrayInterface.parent_type(::Type{<:BitsArray{N, B}}) where {N, B} = Array{UInt8, N}
Base.parent(a::BitsArray) = a.chunks
```

Start by examining the custom BitsArray getindex implementation and the listed ArrayInterface methods, then check how LoopVectorization handles custom array types. Reproduce the attempted integration and determine which interface requirements are missing. Done means establishing whether packed UInt8 elements can be safely supported for vectorized loops, with the required behavior demonstrated by a focused test.

索引モデルが issue の本文から書いたものです。