Need support for `Vector{Int32}` as offsets

[Moelf]

ROOT uses Int32 for all (jagg) offsets internally, everywhere.

[oschulz]

Since Julia defaults to Int for offsets, it might be better to just convert - should be very fast, and it's just a single mem alloc.

[Moelf]

but it is known to be faster to use smaller types in any tight loop. I mean it allocates to do a convert

julia> a = rand(Int32, 200);

julia> function f(ary)
           for _ = 1:100
               convert(Vector{Float64}, ary)
           end
           nothing
       end
f (generic function with 1 method)

julia> @benchmark f($a)
BenchmarkTools.Trial: 10000 samples with 1 evaluation.
 Range (min … max):   8.464 μs … 560.591 μs  ┊ GC (min … max): 0.00% … 93.67%
 Time  (median):     12.672 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   15.314 μs ±  21.938 μs  ┊ GC (mean ± σ):  7.61% ±  5.62%

  ▁▄▅▇█▃                                                       ▂
  ██████▇█▆▇█▄▄▃▄▄▅▅▅▅▃▄▄▄▃▅▅▅▄▃▃▄▄▃▃▄▁▁▃▁▁▁▁▁▁▁▁▁▁▁▅▅▆▆▇▇█▇██ █
  8.46 μs       Histogram: log(frequency) by time      70.4 μs <

 Memory estimate: 176.56 KiB, allocs estimate: 100.

We have to have Int32 vectors, as I said, ROOT forces it, I don't see why we shouldn't support it, if not default to it.

[oschulz]

Hm, yes, let's support Int32 then (withInt as default). May come in handy if the data type is Float32 or Bool.