Make a new AbstractRange that would subset into the pyramid with a given size

From @timholy via Slack:

Hmm, looking at PyramidScheme, do you think there's room for something that brings in fewer dependencies? I was envisioning something like this: img[sizelimit(1000, xrng), sizelimit(1000, yrng)] where sizelimit(maxsz, rng) returns an AbstractRange for which the pyramid has a special slicing implementation. If img isn't a pyramid, then this just returns img[xrng, yrng]. So sizelimit is only a "hint" about what resolution you want for the output.

The core ideas of a slim package would be:

struct SizeLimitedUnitRange <: AbstractUnitRange{Int}
r::UnitRange{Int}
lim::Int
end

Base.iterate(rng::SizeLimitedUnitRange, state...) = Base.iterate(rng.r, state...) # provides fallback behavior for non-pyramids

struct Pyramid{T,N,A<:AbstractArray{T,N},Ax<:NTuple{N,AbstractUnitRange}} <: AbstractArray{T,N}
levels::Vector{A}
axes::Ax
end

function Base.getindex(A::Pyramid, I...)
any(i -> isa(I, SizeLimitedUnitRange), I) || return invoke(getindex, Tuple{AbstractArray, Vararg{Any}}, A, I...)
# implementation that picks the level goes here
end

Start by reading the proposed SizeLimitedUnitRange, sizelimit usage, and Pyramid Base.getindex entry point in the issue. Determine how pyramid levels and axes are represented before implementing the requested size-limited slicing behavior, while preserving the stated fallback for non-pyramids. Done means the new range hint can select an appropriate pyramid level and ordinary array slicing still works.