[NDTensors] Add SortedSets and TagSets

[mtfishman]

Description

This adds SortedSets and TagSets submodules to NDTensors.

SortedSets defines a generic sorted set-like data structure which are more efficient for smaller collections than hash-based sets. Inserting new elements keeps the set sorted (and unique). When combined with SmallVectors from #1202 (using a SmallVector as a data backend for a SortedSet), it should provide very fast data structures for applications like TagSets and QNs. This PR also adds a first draft of a new TagSet type design, which is generic but can make use of SmallVectors and SortedSets to reproduce speed that is similar to the current ITensors.TagSet type but with less code and a simpler, more generic design that will allow users to choose their own tag and storage types.

This is still a draft because I need to test the efficiency of SortedSets (I know there is some part that doesn't fully take advantage of the speed of using a SmallVector but instead uses slower generic code), work out a few more things around the design of the TagSet (and relatedly, QN) type, and add tests and examples.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (ea0f602) 85.41% compared to head (1f49549) 67.37%.

❗ Current head 1f49549 differs from pull request most recent head 069bc70. Consider uploading reports for the commit 069bc70 to get more accurate results

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Additional details and impacted files

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- Coverage   85.41%   67.37%   -18.05%     
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  Files          88       87        -1     
  Lines        8426     8388       -38     
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- Hits         7197     5651     -1546     
- Misses       1229     2737     +1508     

see 36 files with indirect coverage changes

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[mtfishman]

@emstoudenmire I'm going to merge this so we can start testing it out in the wild. You can look at the new TagSet type (just a prototype for now, not yet used in ITensors.jl to replace the current one) to get an idea for how to design a QN type.

Some set operations on SortedSet (and the associated SmallSet, which is a type alias for a SortedSet wrapping a SmallVector) still need to be optimized, but that will be relatively simple. Basically each one (union, intersect, setdiff, symdiff) can take advantage of the data structures being sorted to run in linear time in the length of the sets, but I only implemented that for union and setdiff so far.