eliminate StoredArray

Fixes #6212, #987; also, `UniformScaling` is no longer an `AbstractArray` (#5810).

*Update:* Changed to keep the name `DenseArray`, as there was no consensus for `RegularArray`

~~Also, `BitArray` and `SharedArray` are now subtypes of `AbstractArray`; they are not `RegularArray`s since they don't implement `stride` and don't have the requisite memory layout.  (One of the main purposes of having `RegularArray` as a type is to know the memory layout of `Ptr{T}` conversions, as otherwise it is impossible to safely do a `ccall`.)~~