Sort dictionary keys and set elements in rich output

[jaanos]

This ticket introduces sorting of dictionary keys and set elements (including finite Sets) in rich output, mimicking IPython's behaviour (actually, it uses IPython's _sorted_for_pprint function for sorting).

It also avoids line breaks between the key and value in the LaTeX representation of dictionaries, and introduces sorting in the plain representation of finite Sets.

[jaanos]

In the LaTeX output, the entire contents of the dictionary are now wrapped in big curly braces, as I don't know how to add them only to the first and last line (\left\{ ... \right. will not work, since there's a & in between) - besides, their sizes may not match, which could be ugly. I guess we could have a horizontal layout if the dictionary could fit in a line, but I don't know how to check it (and it may be part of a longer line anyway, thus requiring line breaks).

I will also add sorting for sets. This will require adding a new function to handle Python sets.

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New commits:

​52fba75	Sort dict keys and align when displayed as LaTeX
​ec9f716	Sort dict keys in character art
​f7afdf5	Update doctests

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​1d742d5	Sort set elements in rich representation
​dfb105e	Fix representations of dicts and sets
​2aeb5ce	Fix grammatical errors in docstrings
​a03bf7a	Sort and add rich representation for finite Sets

[jaanos]

I have now reverted the layout changes in LaTeX representation of dictionaries, instead wrapping key-value pairs in {...}, as suggested by Aram. Sorting is now also applied to Python's sets and frozensets and Sage's finite Sets (for which the representation is the same as for sets, except when the Set is empty).

[chapoton]

red branch

[embray]

Ticket retargeted after milestone closed

[mkoeppe]

Batch modifying tickets that will likely not be ready for 9.1, based on a review of the ticket title, branch/review status, and last modification date.