pkg/math: added MultipleOf

Also:
- add support for Decimal type (internal)
- add correspondng support in openapi package

Change-Id: Idc4b829423bba63380a8edfc1109c345f4248355
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2640
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi_test.go b/encoding/openapi/openapi_test.go
index b085f75..430b5d4 100644
--- a/encoding/openapi/openapi_test.go
+++ b/encoding/openapi/openapi_test.go
@@ -55,6 +55,10 @@
 		"strings.json",
 		defaultConfig,
 	}, {
+		"nums.cue",
+		"nums.json",
+		defaultConfig,
+	}, {
 		"oneof.cue",
 		"oneof.json",
 		defaultConfig,