remove unused classes/functions from dictutil.py

[warner]

I was trying to enhance our unit tests to get full+stable coverage of everything in dictutil.py, when I discovered:

• we aren't actually using most of it
• I could replace one large class with collections.defaultdict and itertools.count
• most of the functionality has since been subsumed by python builtins

So this patch deletes most of dictutil.py and the matching tests. Yay negative code days!

[codecov-io]

Codecov Report

Merging #438 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #438      +/-   ##
==========================================
- Coverage   88.06%   87.89%   -0.17%     
==========================================
  Files         148      148              
  Lines       27954    27553     -401     
  Branches     3986     3953      -33     
==========================================
- Hits        24617    24219     -398     
+ Misses       2624     2623       -1     
+ Partials      713      711       -2

Impacted Files	Coverage Δ
src/allmydata/util/dictutil.py	100% <ø> (+1.62%)	⬆️
src/allmydata/util/nummedobj.py	66.66% <100%> (-1.34%)	⬇️
src/allmydata/stats.py	85.34% <0%> (-0.87%)	⬇️
src/allmydata/immutable/upload.py	95.25% <0%> (-0.28%)	⬇️
src/allmydata/web/status.py	82.86% <0%> (-0.23%)	⬇️
src/allmydata/mutable/servermap.py	94.19% <0%> (+0.48%)	⬆️

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[meejah]

👍 LGTM