keys/LinearIndices for non-Int indices

[sostock]

Due to LinearIndices only supporting Ints, keys and LinearIndices do not work when the indices are not representable as Ints:

julia> keys(1:typemax(Int128))
ERROR: InexactError: trunc(Int64, 170141183460469231731687303715884105727)
[...]

julia> LinearIndices(1:typemax(Int128))
ERROR: InexactError: trunc(Int64, 170141183460469231731687303715884105727)
[...]

Related: #27090

[mbauman]

Yeah, this is a bit of a wart. I have an old branch where I started parameterizing all these indices structures, but I recall hitting a number of roadblocks making this trickier than it first appears.

In some senses, I'm ok with LinearIndices being restricted to only being integer values — you're explicitly requesting a "memory offset"-like representation of the array, so an inability to represent values larger than Int is reasonable. The trouble is that keys should definitely always work, and right now there's a nice symmetry that it always returns either a Linear- or CartesianIndices.

The easy fix is to just patch up keys(::AbstractRange) to deal with these cases, but it'd be nice to find a generic behavior that's a bit more friendly to non-Int indices.