Make `blocks(randn(2, 2))[1, 1]` return the original array

[mtfishman]

Currently, blocks has this behavior:

julia> using BlockArrays

julia> blocks(randn(2, 2))[1, 1]
2×2 view(::Matrix{Float64}, BlockSlice(Block(1),Base.OneTo(2)), BlockSlice(Block(1),Base.OneTo(2))) with eltype Float64:
 -1.50189  2.04898
  1.76046  0.382146

julia> using Pkg; Pkg.status("BlockArrays")
Status `~/Simons Foundation Dropbox/Matthew Fishman/Documents/workdir/BlockArrays.jl/Project.toml`
  [8e7c35d0] BlockArrays v1.0.1

I think it could be nice to have it return the input array instead of a view when it can be known from the type of the original array that it only has one block. Working with the view, while not too bad, is a bit more complicated than just working with the original array.

A design could be an alternative type to BlocksView:

struct OneBlockView{T,N,A<:AbstractArray{T,N}} <: AbstractArray{T,N}
    array::A
end
Base.size(a::OneBlockView) = ntuple(Returns(1), ndims(a))
function Base.getindex(a::OneBlockView{T,N}, i::Vararg{Int,N}) where {N}
    @boundscheck checkbounds(a, i...)
    return a.array
end
function Base.setindex!(a::OneBlockView{T,N}, b, i::Vararg{Int,N}) where {N}
    @boundscheck checkbounds(a, i...)
    copyto!(a.array, b)
    return a
end

blocks(a::Array) = OneBlockView(a)

It may warrant having an is_blocked trait (say to detect if wrapped arrays are blocked or not) but I think that is a different story.