FoldsThreads: Extra threaded executors for JuliaFolds/*.jl

FoldsThreads.jl provides extra thread-based executors usable with various JuliaFolds/*.jl packages such as Transducers.jl and FLoops.jl.

                                  Executors
                           ,----------------------.
     Algorithms            |    FoldsThreads.jl    |         Data structures
,------------------.       |-----------------------|       ,-----------------.
|  FLoops,         |       |  ThreadedEx*          |       |  Array,         |
|  Folds,          |       |  WorkStealingEx,      |       |  Tables,        |
|  Transducers,    |  ---  |  DepthFirstEx,        |  ---  |  FGenerators,   |
|  OnlineStats,    |       |  TaskPoolEx,          |       |  Dict,          |
|  DataTools, ...  '       |  NondeterministicEx,  |       |  Set, ...       |
`------------------'       |  ...                  |       `-----------------'
                           `-----------------------'

(* ThreadedEx is the default executor provided by Transducers.jl)

• WorkStealingEx implements work stealing (continuation stealing). Useful for load-balancing.
• DepthFirstEx implements depth-first scheduling. Useful for findfirst-type computations.
• TaskPoolEx: Task pool executor. Useful for fine execution control (e.g., back pressure and "background" threads).
• NondeterministicEx: An executor for parallelizing computations with non-parallelizable iterators.