lang: Map Go struct fields using lang struct tag

Converting a reflect.Type of KindStruct did not respect the `lang` tag
on struct fields incidating how fields from mcl structs should be mapped
even though resource field names did. This patch should allow structs
with mapped fields to be respected.

Signed-off-by: Joe Groocock <me@frebib.net>

lang/types/type.go

@@ -150,9 +150,16 @@ func TypeOf(t reflect.Type) (*Type, error) {
 				return nil, err
 			}
 			// TODO: should we skip over fields with field.Anonymous ?
-			m[field.Name] = tt
-			ord = append(ord, field.Name) // in order
-			// NOTE: we discard the field.Tag data
+
+			// TODO: make struct field name lookup consistent with a helper function
+			// if struct field has a `lang:""` tag, use that instead of the struct field name
+			fieldName := field.Name
+			if alias, ok := field.Tag.Lookup(StructTag); ok {
+				fieldName = alias
+			}
+
+			m[fieldName] = tt
+			ord = append(ord, fieldName) // in order
 		}
 
 		return &Type{