lib/types.nix: Disable 32-bit integer types for now

This file doesn't evaluate in 32-bit versions of Nix because the integer
type is a signed 32-bit integer there, so 4294967296 causes an 'invalid
integer' error.  I see no other way around than commenting this out :(

(s32 could be made to work by tweaking the expressions a bit, but didn't
do that for now since it'd be asymmetric to have s32 but no u32).