[libsmi] preserving comments

Hi Frank,

My understanding is that in the current implementation comments are not being preserved at any data structure in libsmi.

If I'm right, I would like to suggest that a code will be added to do this. The advantages of preserving comments are that utilities like dumpsmi will be able to regenerate the exact contents of the original MIB module, and that there are some places, like enum values definition, were it is more convinient to use a comment near the definition.

Mark.