Differentiate elements skipped during parsing and comment processing

The lists of elements skipped during parsing and comment processing need
to differ. Currently there are two reasons:

  * When parsing, we want to skip <yconst>, because it's just a useless
    wrapper. But when processing comments we don't want to skip it,
    because this is the element to which comments are attached to for
    various literals.

  * When parsing, we don't want to skip <element>, because we need to
    handle it specially in case it's inside <map>. But when processing
    comments it's just a useless wrapper.

Fixes #32.