Add is_strided

[oxinabox]

see JuliaLang/julia#2345 (comment)

[ChrisRackauckas]

Definitely agree

[chriselrod]

I want something that only returns true if we can call pointer (or Base.unsafe_convert(Ptr{T}, A)) and get a Ptr to valid CPU memory.
With it returning false for GPUArrays (which are <: StridedArray), I could use that for LoopVectorization.check_args.

Would we want separate is_strided and is_cpu_strided (maybe adding is_device_strided as a mirror)?

[chriselrod]

@mbauman, I hope I can ping you here to ask both about implementation, and what you think should be covered.

I would be happy with something like:

is_strided(::Any) = false
is_strided(::StridedArray) = true
# library support
is_strided(::GPUArrays.GPUArray) = false
is_strided(::StaticArrays.MArray) = true

and I can special case support for things like PermutedDimsArray of is_strided arrays myself.

But it depends upon what interfaces we want is_strided(A) == true to promise support for.
What I am suggesting above is:

1. You can get a pointer to memory the CPU can work with.
2. The memory is strided and column major.

I'd also like to have something else for whether isone(LinearAlgebra.stride1(A)) is known to be true at compile time.

[Tokazama]

Can we close this now that we have defines_strides?