pkg/strings: add MinRunes and MaxRunes

Also adds respective conversions for the OpenAPI
encoding.

cue/builtins.go is generated using go generate

Change-Id: I3b709cb56bd59fbe08a0376d4b4760b9b21a2e3a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2625
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi_test.go b/encoding/openapi/openapi_test.go
index d5eb721..b085f75 100644
--- a/encoding/openapi/openapi_test.go
+++ b/encoding/openapi/openapi_test.go
@@ -51,6 +51,10 @@
 		"array.json",
 		defaultConfig,
 	}, {
+		"strings.cue",
+		"strings.json",
+		defaultConfig,
+	}, {
 		"oneof.cue",
 		"oneof.json",
 		defaultConfig,
@@ -82,6 +86,9 @@
 			inst := cue.Build(load.Instances([]string{filename}, nil))[0]
 
 			b, err := Gen(inst, tc.config)
+			if err != nil {
+				t.Fatal(err)
+			}
 			var out = &bytes.Buffer{}
 			_ = json.Indent(out, b, "", "   ")