mapreduce fallback tracking issue #246
Description
in trying to understand why count was slow, i came across a comment explaining that it used to fallback to mapreduce but now does not because of internal refactorings of the API. so i dug through this a bit so as to learn how DiskArrays worked, and found that the Generator is hit instead of mapfoldl_impl at least back to julia v1.9. didn't go back further as that is the current minimum version required by DiskArrays.jl.
given enough developer time, it'd be nice to have these fallback method work again. in the meantime, we might consider blocking them out with VERSION as is commonly done in CUDA.jl. probably shouldn't delete them entirely (yet), as they would serve as a template for updating them.