Improved dict printing

This is an 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.

This PR keeps the old behavior (mostly) as `showcompact`. I then added new
functionality in `show` and `showlimited` for Associative and
Key/ValueIterators, printing newlines as delimiters between key/value pairs.
When output is limited, keys are truncated to the left third of the TTY screen
and values are truncated at newlines or the screen edge, with the `=>`
separators aligned.
Merge pull request #5706 from mbauman/showdict

RFC: Moderately improved dict printing