Only implements the different conversion types from and to ints for now.
Unfortunately very reliant on builtins.{floor,ceil} which can't be
implemented purely except very inefficiently (to my knowledge), so it
only really works for C++ Nix >= 2.4. Tests are thus skipped for
C++ Nix 2.3.
Change-Id: Idcb1a11df11e214cdba3f2a0715472b370daa7dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9008
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
