Add native int128/uint128 support

[lukas-weber]

I saw in #2154 that native float16 support is planned. Are there plans to add int128 as well?

[jhendersonHDF]

Hi @lukas-weber,

we haven't yet considered adding support for int128 and uint128 at this time, but you should be able to work around that for the time being (if you aren't already) by defining your own datatype and conversion routines.

[derobins]

@lukas-weber - Do you have a specific use case in mind?

[lukas-weber]

Apologies for the late answer. Admittedly my use case was esoteric: saving the internal state of a random number generator. I was doing it using some automatic serialization code in Julia, then found out it croaked on the Int128 numbers, which are not supported by HDF5.jl.

I found JuliaIO/HDF5.jl#301 which mentioned adding support on their end as a custom datatype on a similar footing with adding Float16 support. But the reasoning was to wait until Float16 support was adopted upstream. So I was curious is a similar thing was happening for Int128.

[derobins]

We might perform additional datatype rework over the summer, so I'm tentatively assigning this to 1.14.5