container/dictionary.[ch]: fix numerous small issues

container/dictionary.[ch]: fix numerous small issues

The 4th diagnostic in particular is interesting because it is
located in unreachable code, so simply remove the entire
conditional block which is always false.

The 5th diagnostic is useful: it illuminated a missing function
declaration in the corresponding header, which was added.

Fixes compiler diagnostics:

dictionary.c:236:1: warning: no previous prototype for function
    'mowgli_dictionary_retune' [-Wmissing-prototypes]

dictionary.c:335:1: warning: no previous prototype for function
    'mowgli_dictionary_link' [-Wmissing-prototypes]

dictionary.c:414:1: warning: no previous prototype for function
    'mowgli_dictionary_unlink_root' [-Wmissing-prototypes]

dictionary.c:758:78: warning: cast from 'const void *' to 'void *'
    drops const qualifier [-Wcast-qual]

dictionary.c:846:1: warning: no previous prototype for function
    'mowgli_dictionary_size' [-Wmissing-prototypes]