Introduce AbstractSparseArray

[lindahua]

Sparse representation is also useful for arrays of other ranks (e.g. sparse vector or higher dimensional sparse arrays).

My proposal is to

abstract AbstractSparseArray{Tv, Ti<:Integer, N} <: StoredArray{Tv, N}

typealias AbstractSparseVector{Tv, Ti<:Integer} AbstractSparseArray{Tv, Ti, 1}
typealias AbstractSparseMatrix{Tv, Ti<:Integer} AbstractSparseArray{Tv, Ti, 2}

This does't break any existing codes. But it allows future generalization of sparse representation to generic arrays, and allows packages to explore such.

[lindahua]

In some of my applications, sparse vectors can be useful (e.g. representing a word histogram of a document based on a large vocabulary).

[jiahao]

I think we may need this anyway if we want to do implement something like CSR matrices as suggested in #5932.

[stevengj]

@jiahao, we already have AbstractSparseMatrix. It makes conceptual sense to me to generalize this to AbstractSparseArray, however, since you want to include sparse multilinear operators etc, and it parallels AbstractArray/AbstractMatrix etc.