1d set/getindex for abstract sparse

ref #7023

This implements `setindex!` and `getindex` with 1d and boolean indices for `AbstractSparseMatrix`. An optimized implementation for `SparseMatrixCSC` can be done separately.
Will check that. There are a few other places in `sparsematrix.jl` where `searchsorted` can be used. Shall replace them as well if there's no performance impact.