`Int8` Offsets

Hello, first of all, thanks for the beautiful package. Over at Oceananigans.jl and ClimaOcean.jl, we use offset arrays to store the underlying data of our Field types. However, now that we are increasing the complexity of the model, we are having increasingly frequent problems with GPU parameter space when launching kernels that require many offset array inputs.

We think that this has to do with the fact that offsets are Int64 types, which consume quite a lot of parameter space, however, we typically need offsets in the range of -10 .. 10, so Int8 would be enough for our scope.

However, it looks like the Int type is hardcoded in the OffsetArray type.
https://github.com/JuliaArrays/OffsetArrays.jl/blob/5972e9e7fa148e87f127cc2b0152bce1e94dfd16/src/OffsetArrays.jl#L114
Is there a specific reason for having offsets as Int64s or would it be possible to allow flexibility in the sizes of the offsets?

In the latter case, could I attempt a PR allowing lower-sized offsets?

cc @glwagner

Start in src/OffsetArrays.jl at the OffsetArray definition linked in the issue, and review the existing discussion about why offsets use Int. Determine how allowing smaller integer offset types would affect the package's current indexing behavior and GPU parameter use. Done means lower-sized offsets are supported where appropriate without breaking existing offset-array behavior.