engine, lang: Improve send/recv significantly

Part of this was rotten, and not fully functional. This fixes the rot,
adds some tests, and improves the type checking that occurs when sending
and receiving values. In addition, a significant portion of this happens
at compile time.

There is still more work to be done here, but this should get us a good
chunk of the way for now.

engine/graph/sendrecv.go

@@ -22,6 +22,7 @@ import (
 	"reflect"
 
 	"github.com/purpleidea/mgmt/engine"
+	engineUtil "github.com/purpleidea/mgmt/engine/util"
 
 	multierr "github.com/hashicorp/go-multierror"
 	errwrap "github.com/pkg/errors"
@@ -47,16 +48,51 @@ func (obj *Engine) SendRecv(res engine.RecvableRes) (map[string]bool, error) {
 			st = v.Res.Sent()
 		}
 
+		if st == nil {
+			e := fmt.Errorf("received nil value from: %s", v.Res)
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+
+		if e := engineUtil.StructFieldCompat(st, v.Key, res, k); e != nil {
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+
 		// send
+		m1, e := engineUtil.StructTagToFieldName(st)
+		if e != nil {
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+		key1, exists := m1[v.Key]
+		if !exists {
+			e := fmt.Errorf("requested key of `%s` not found in send struct", v.Key)
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+
 		obj1 := reflect.Indirect(reflect.ValueOf(st))
 		type1 := obj1.Type()
-		value1 := obj1.FieldByName(v.Key)
+		value1 := obj1.FieldByName(key1)
 		kind1 := value1.Kind()
 
 		// recv
+		m2, e := engineUtil.StructTagToFieldName(res)
+		if e != nil {
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+		key2, exists := m2[k]
+		if !exists {
+			e := fmt.Errorf("requested key of `%s` not found in recv struct", k)
+			err = multierr.Append(err, e) // list of errors
+			continue
+		}
+
 		obj2 := reflect.Indirect(reflect.ValueOf(res)) // pass in full struct
 		type2 := obj2.Type()
-		value2 := obj2.FieldByName(k)
+		value2 := obj2.FieldByName(key2)
 		kind2 := value2.Kind()
 
 		if obj.Debug {