Add trait for mmap-able?

[oxinabox]

c.f. https://github.com/JuliaData/CSV.jl/pull/511/files#r332346349

Note that some FilePaths can be mmaps directly, in particular SystemPaths
other cannot, like AWS3.S3Paths
But I guess that can be fairly easily handled by treating SystemPaths like strings,
and all other AbstractPaths like Cmds etc.

This would be a nice usecase for JuliaLang/julia#32732
when I eventually make that work,
since then can just compile-time know if something overload's hasmethod(MMap.mmap, ...)

OTOH, I suspect that only SystemPath's are ever going to be mmap-able.
So maybe htis don't need to be extensable,
and existing differentiation between SystemPaths and all other paths is enough.
If so, we can close this

[rofinn]

Hmmm, looks like this may be a requirement to have filepaths work with JLD2.jl? I think it might make sense to have a fallback for non-system paths where we download non-system paths locally and then call MMap.map?

[rofinn]

For now I think we’re just going to support mmap for SystemPaths. If folks want to consistently mmap files then they can cp the file to a temp directory and mmap that.