cl: Several fixes

• Use wrapper types calling translation functions instead of TRANSLATE-{FROM,TO}-FOREIGN methods as they seem not to be called in some cases.
• Use the (:STRUCT SOME-C-STRUCT) notation instead of the deprecated direct reference to SOME-C-STRUCT.
• Add missing values in enums and bit fields.
• Use cffi-grovel to define system types (SIZE-T, OFF-T, etc).
• Wrap GPGME-DATA-T in a class (like contexts).
• Use the FINALIZE function from trivial-garbage to free the C objects for contexts, keys and data automatically.
• Make DATA-READ-CB and DATA-WRITE-CB run faster.
• Update the README file.

• Signed-off-by: Guillaume LE VAILLANT <glv@posteo.net>