make keys(dict) and values(dict) more useful

[StefanKarpinski]

Currently the very common operation of iterating a Dict's keys in sorted order requires writing the overly verbose sort!(collect(keys(d))). This could be abbreviated to sort(keys(d)) by adding a sort method for KeyIterator, but do we manually add methods for all relevant functions in Base? What about user-defined functions that make sense for general collection-like types? At some point (IIRC) I proposed making KeyIterator and ValueIterator subtypes of AbstractVector, which would make many things "just work". The trouble is that indexing is O(n); however, I believe that something like @JeffBezanson's ordered dict experiment could make indexing O(1).

[StefanKarpinski]

Mutating keys(d) and values(d) could actually potentially make sense – keys(d)[i] = k and values(d)[i] = v could be efficient equivalents of these:

# keys(d)[i] = k
d[k] = pop!(d, collect(d)[i].first)

# values(d)[i] = v
d[collect(d)[i].first] = v

Moreover, if dictionaries are ordered, then writing code like sort!(keys(d)) could be another way of writing sort!(d, by=p->p[1]) and similarly sort!(values(d)) could be equivalent to doing sort!(d, by=p->p[2]), assuming sort! for Dicts is allowed (questionable since indexing Dicts can't be).

[ararslan]

Many moons ago I had a PR for a sort fallback that would handle general iterables (#16853), though it was somewhat ill-conceived at the time. The idea was to just collect before sorting.

[JeffBezanson]

Dup of #10092 (almost exactly 2 years later!) ?

[rfourquet]

I would often want to call also sort(::Dict) at the REPL, which @ararslan PR would allow.

[ararslan]

Wouldn't you want sort(::Dict) to give you a SortedDict though? 🙂

[rfourquet]

Like I said in your PR, I would prefer the predicability of getting an array out. If I wish a SortedDict, SortedDict(::Dict) is simple enough! As for my visual consumption at the REPL, an array of pairs is plenty enough :)

[mbauman]

We probably don't need both of these open. Closing in favor of #10092.