RFC: Moderately improved dict printing

This is a modest attempt at improving the situation for issue #1759.  I've added
slightly enhanced `summary`s with information about the number of k/v pairs for
Associative and Key/ValueIterator types.

I'm still slightly confused by all the different methods used for show, but
this PR keeps the old behavior (mostly) as `showcompact`. I then added new
functionality in `show` and `showlimited`, using newlines as delimiters between
key/value pairs.  When output is limited, values are truncated at newlines or
the TTY screen edge and a limited number of pairs are printed.

The key and value iterators no longer spit out entire dictionaries when shown.
They instead show a limited `{}` array of the keys and values.