Instead, add diameter_make:format_error/1 to allow the caller to format
if desired, which is what applications like compiler and yecc do. Use
this to check that the expected error is the one actually generated in
the compiler suite.

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.

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

"spec" is an old term the internal representation of a dictionary. The
new extensions are in the style or those that compile(3) can generate.