Refactoring: Add AbstractSparseMatrixCSC

[tkf]

Now that we have accessor-based code to manipulate sparse arrays #32953, we can expose the rich set of functions in SparseArrays.jl in a highly extensible manner by "just" replacing SparseMatrixCSC with AbstractSparseMatrixCSC in appropriate places. This is the second step I discussed with @ViralBShah in #30173 (comment).

Quick summary

This patch introduces a new abstract type AbstractSparseMatrixCSC

julia/stdlib/SparseArrays/src/abstractsparse.jl

Lines 28 to 30 in 89a0c1a

	AbstractSparseMatrixCSC{Tv,Ti<:Integer} <: AbstractSparseMatrix{Tv,Ti}
	Supertype for matrix with compressed sparse column (CSC).

and then declare SparseMatrixCSC to be the subtype of it

julia/stdlib/SparseArrays/src/sparsematrix.jl

Line 19 in 89a0c1a

struct SparseMatrixCSC{Tv,Ti<:Integer} <: AbstractSparseMatrixCSC{Tv,Ti}

I then apply a series of diffs like below so that the functions previously dispatched on SparseMatrixCSC is now dispatched on AbstractSparseMatrixCSC

@@ -31,7 +31,7 @@ end
 # In matrix-vector multiplication, the correct orientation of the vector is assumed.
 const AdjOrTransStridedMatrix{T} = Union{StridedMatrix{T},Adjoint{<:Any,<:StridedMatrix{T}},Transpose{<:Any,<:StridedMatrix{T}}}
 
-function mul!(C::StridedVecOrMat, A::SparseMatrixCSC, B::Union{StridedVector,AdjOrTransStridedMatrix}, α::Number, β::Number)
+function mul!(C::StridedVecOrMat, A::AbstractSparseMatrixCSC, B::Union{StridedVector,AdjOrTransStridedMatrix}, α::Number, β::Number)
     size(A, 2) == size(B, 1) || throw(DimensionMismatch())
     size(A, 1) == size(C, 1) || throw(DimensionMismatch())
     size(B, 2) == size(C, 2) || throw(DimensionMismatch())

[ViralBShah]

This also largely looks ok to me. Should be completely non-breaking, so can merge when we are ready.

[tkf]

I'd appreciate it if this PR can be merged relatively quickly to avoid interference with other PRs. I opened PR #33054 for finalizing the interface definition and prefer to do a discussion on the interface there (including naming bikeshedding (if we have to...)). Note that renaming AbstractSparseMatrixCSC afterward should be very easy since this name is unique in this repository.