Proposal: sparse overlay

[putianyi889]

Motivation: JuliaSparse/SparseArrays.jl#512

Solution (just a brief sketch):

struct SparseOverlay{T,N,D<:AbstractArray{T,N}} <: AbstractArray{T,N}
    data::Dict{CartesianIndex{N},T}
    base::D
end
const SparseArray{T,N} = SparseOverlay{T,N,FillArrays.Zeros{T,N}}

@inline function Base.getindex(a::SparseOverlay{T,N}, I::CartesianIndex{N}) where {T,N}
    @boundscheck checkbounds(a, I)
    return get(a.data, I, a.base[I])
end