Consider allowing for unsigned 64-bit masks

[glopesdev]

Currently the generators are limited to signed 32-bit mask values, which means 64-bit payload values cannot be packed correctly:

https://github.com/harp-tech/reflex-generator/blob/de85b82dac0a120d218b7b8b47b6feb8e74be1ff/interface/Interface.tt#L241-L241