Make use of const variant of asn1_node
Add type 'asn1_node_const' and apply it to function params where possible.
This allows gnutls!1000 (merged) to be worked on easier.
- Code modified for feature
- There is a test suite reasonably covering new functionality or modifications
- Function naming, parameters, return values, types, etc., are consistent with other code
- This feature/change has adequate documentation added
- No obvious mistakes in the code