Greatly improve performance of sorting dictionaries

[tgoyne]

I noticed in #5163 that dictionary sorting is rather slow and took a stab at improving it.

Dictionary lookup by index is slow, so read all of the keys/values from the dictionary into a vector and sort that instead. Using a dictionary iterator to read the values replaces O(N log N) ClusterTree lookups with O(log N), so this is asymptotically faster. The benchmark is sped up from 3.77 seconds to 21.62 milliseconds, but more reasonably sized Dictionaries will see proportionally smaller benefits.

The downside of this is that the array of Mixed requires 24 bytes of scratch space per element in the dictionary. We already require 8 bytes per element to store the results, so this is just a constant factor increase rather than an asymptotic change in memory use and it's probably not a problem.

[bmunkholm]

@tgoyne @jedelbo Any downsides to getting this reviewed and merged?

[tgoyne]

#5780 might make this unnecessary as it significantly changes the performance characteristics of dictionaries. If this is still a meaningful gain then there's probably no downside; the increased scratch memory usage really isn't signficant.

[tgoyne]

Still seems worth doing:

next-major:
Req runs:    5  SortIntList (MemOnly, EncryptionOff):         min 179.36ms     max 182.32ms     median 180.32ms     avg 180.77ms     stddev   1.46ms
Req runs:    5  SortIntDictionary (MemOnly, EncryptionOff):   min 687.99ms     max 702.65ms     median 692.61ms     avg 693.63ms     stddev   5.79ms

tg/dict-sort:
Req runs:    5  SortIntList (MemOnly, EncryptionOff):         min 190.65ms     max 202.14ms     median 201.05ms     avg 197.78ms     stddev   5.46ms
Req runs:   10  SortIntDictionary (MemOnly, EncryptionOff):   min  46.27ms     max  48.36ms     median  47.08ms     avg  47.26ms     stddev    762us

next-major + tg/dict-sort:
Req runs:    5  SortIntList (MemOnly, EncryptionOff):         min 182.83ms     max 194.63ms     median 191.08ms     avg 189.38ms     stddev   4.59ms
Req runs:   14  SortIntDictionary (MemOnly, EncryptionOff):   min  33.83ms     max  37.43ms     median  34.86ms     avg  35.24ms     stddev   1.13ms

[jedelbo]

Great! Let's do it.