deprecate ClassifierCollection

[oxinabox]

First part of #496

I don't know what to recommend using instead.
Probably a DefaultDict(Collection)?

[codecov]

Codecov Report

Merging #524 into master will decrease coverage by 5.51%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   93.39%   87.88%   -5.52%     
==========================================
  Files          32       32              
  Lines        2544     2105     -439     
==========================================
- Hits         2376     1850     -526     
- Misses        168      255      +87

Impacted Files	Coverage Δ
src/classified_collections.jl	75% <100%> (-15%)	⬇️
src/delegate.jl	50% <0%> (-41.67%)	⬇️
src/container_loops.jl	27.5% <0%> (-23.72%)	⬇️
src/heaps/minmax_heap.jl	91.76% <0%> (-8.24%)	⬇️
src/heaps.jl	92% <0%> (-8%)	⬇️
src/heaps/arrays_as_heaps.jl	86.2% <0%> (-7.74%)	⬇️
src/multi_dict.jl	67.46% <0%> (-7.54%)	⬇️
src/tokens2.jl	92.59% <0%> (-7.41%)	⬇️
src/sorted_set.jl	82.94% <0%> (-6.82%)	⬇️
... and 22 more

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[oxinabox]

@kmsquire can I request a review?
I would like to get this into 0.17.x so can remove in 0.18

[kmsquire]

One suggestion, but I have no problem with the depreciation.