feat(sterni/nix): add trivial float library

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

users/sterni/nix/float/default.nix

@@ -0,0 +1,23 @@
+{ depot, ... }:
+
+let
+  inherit (depot.users.sterni.nix)
+    num
+    ;
+in
+
+rec {
+  # In C++ Nix, the required builtins have been added in version 2.4
+  ceil = builtins.ceil or (throw "Nix implementation is missing builtins.ceil");
+  floor = builtins.floor or (throw "Nix implementation is missing builtins.floor");
+
+  truncate = f: if f >= 0 then floor f else ceil f;
+  round = f:
+    let
+      s = num.sign f;
+      a = s * f;
+    in
+    s * (if a >= floor a + 0.5 then ceil a else floor a);
+
+  intToFloat = i: i * 1.0;
+}