feat(nix/yants): add unit type

Has one less value than bool.

Change-Id: I2f2db07b1eb1f49172942bc1d8c6ceb30d987d84
Reviewed-on: https://cl.tvl.fyi/c/depot/+/661
Reviewed-by: tazjin <mail@tazj.in>

nix/yants/default.nix

@@ -93,6 +93,7 @@ with builtins; let
 in lib.fix (self: {
   # Primitive types
   any      = typedef "any" (_: true);
+  unit     = typedef "unit" (v: v == {});
   int      = typedef "int" isInt;
   bool     = typedef "bool" isBool;
   float    = typedef "float" isFloat;

nix/yants/tests/default.nix

@@ -9,6 +9,7 @@ with depot.nix.yants;
 deepSeq rec {
   # Test that all primitive types match
   primitives = [
+    (unit {})
     (int 15)
     (bool false)
     (float 13.37)