That is, preprocessed forms that can be passed to compile:forms/1,2.

To set @avp_vendor_id, @codecs and @custom_types as required for
imported avps.

The intention was that the type would represent an improper list whose
head was an integer and whose tail was an orddict but that doesn't seem
to be dialyzer's interpretation anyway.

In particular, make codec/2 flexible as to what's generated, the formats
(erl, hrl, parse, forms and beam) being passed in the options list and
defaulting to [erl, hrl]. The 'parse' format is the internal format to
which dictionaries are parsed, which can be manipulated by flatten/1
before being passed back to codec/2 or format/1.

Remove the (undocumented) dict/1,2 since codec/2 now subsumes it with
the 'parse' option.

The latter is now unnecessary given that codec/2 can return a parse and
format/1 can return the dictionary format.

Function can now take a literal dictionary as input, instead of a path,
and can return results instead of writing them to the filesystem.

Didn't matter before diameter_make since the module was only called from
diameterc(1).

Instead of being output as a consequence of a debug option.

In preparation for allowing return instead of write.