pkg/struct: add Min/MaxFields builtins

- also define their mappings to OpenAPI

Issue #56

Change-Id: I3db075ea25d5e5df8bf3a003fd881862e73be683
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2685
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi_test.go b/encoding/openapi/openapi_test.go
index 75e9fcc..9fbe314 100644
--- a/encoding/openapi/openapi_test.go
+++ b/encoding/openapi/openapi_test.go
@@ -51,6 +51,10 @@
 		"array.json",
 		defaultConfig,
 	}, {
+		"struct.cue",
+		"struct.json",
+		defaultConfig,
+	}, {
 		"strings.cue",
 		"strings.json",
 		defaultConfig,