OpenPGP certificate/key management thoughts about function re-organization and cleanup

Hey folks--

This message describes some thoughts about the GnuTLS OpenPGP API (in
particular, the bits that used to be OpenCDK).  These thoughts have
come after i've played with the library for a while, but they may
still be ill-informed.  I'd be happy to have flaws in my reasoning
pointed out.  My first observation was dismay at just how many
functions were specifically available for the OpenPGP key management
functionality.  The OpenPGP function reference [0] is enormous and
difficult to wade through.  Furthermore, much functionality seems
duplicated (or quadruplicated!) across different functions, which can
make it difficult to write clean, simple code with the library.

What follows are some proposals that might simplify and streamline the
set of functions available, which would make it easier for a skimming
developer to get a sense of the main functionality offered by the
library, and would facilitate writing tighter code in scenarios where
certain aspects might be better treated as abstractions.

Primary Keys vs. Subkeys
------------------------

There are currently a lot of duplicated functions which deal with the
subkey and primary keys differently.  For example:

 gnutls_openpgp_crt_get_pk_algorithm()
 gnutls_openpgp_crt_get_subkey_pk_algorithm()

These behave exactly the same way, but the primary key is sort of
special-cased out.