refactor:

- merge xmldsig and xmlenc into a single package
- implement thread-safe error capture
- implement encryption using libxmlsec, discarding the go implementation

error_thunk.go

@@ -0,0 +1,6 @@
+package xmlsec
+
+// void onError_cgo(char *file, int line, char *funcName, char *errorObject, char *errorSubject, int reason, char *msg) {
+//   onError(file, line, funcName, errorObject, errorSubject, reason, msg);
+// }
+import "C"