move the checked_dims implementation to a dependency package?

[nsajko]

The functionality is required for all packages that require dense in-memory arrays, I think. There's also a Julia feature request to provide the functionality with Base: JuliaLang/julia#54327. Why not just make a small package for this? Here's a proposal, I'll add permissions to the two of you and request a move to JuliaArrays: https://github.com/nsajko/CheckedSizeProduct.jl EDIT: moved to https://github.com/JuliaArrays/CheckedSizeProduct.jl

Furthermore, AFAIK checked_dims is the only piece of implementation that FixedSizeBitArray shares with FixedSizeArray, so, assuming collect_as also becomes owned by another package, FixedSizeBitArrays.jl seems like it could stand on its own as yet another package. Cross-referencing PR #75.