interim release to help with future binary compatibility

[kevinushey]

This PR sets the ground-work for an RcppParallel 5.1.10 release, whose main goals are to remove the accidental direct dependency on TBB that packages get when using RcppParallel's utilities. Using gaston from CRAN as an example:

kevin@kevinushey-NPCV:~/Library/R/arm64/4.4/library
$ nm gaston/libs/gaston.so | grep tbb | grep U | c++filt
                 U tbb::interface5::internal::task_base::destroy(tbb::task&)
                 U tbb::interface7::internal::task_arena_base::internal_terminate()
                 U tbb::interface7::internal::task_arena_base::internal_initialize()
                 U tbb::interface9::global_control::internal_create()
                 U tbb::interface9::global_control::internal_destroy()
                 U tbb::task_group_context::cancel_group_execution()
                 U tbb::task_group_context::register_pending_exception()
                 U tbb::task_group_context::init()
                 U tbb::task_group_context::reset()
                 U tbb::task_group_context::~task_group_context()
                 U tbb::task::note_affinity(unsigned short)
                 U tbb::internal::throw_exception_v4(tbb::internal::exception_id)
                 U tbb::internal::get_initial_auto_partitioner_divisor()
                 U tbb::interface7::internal::task_arena_base::internal_execute(tbb::interface7::internal::delegate_base&) const
                 U tbb::task_group_context::is_group_execution_cancelled() const
                 U tbb::internal::allocate_child_proxy::allocate(unsigned long) const
                 U tbb::internal::allocate_continuation_proxy::allocate(unsigned long) const
                 U tbb::internal::allocate_root_with_context_proxy::free(tbb::task&) const
                 U tbb::internal::allocate_root_with_context_proxy::allocate(unsigned long) const
                 U typeinfo for tbb::task

What this means is that any attempts to RcppParallel which change tbb internals will break packages like gaston, since those symbols may no longer be available (e.g. because they've moved to different namespaces, or APIs have changed altogether).

This PR resolves the issue by hiding the implementation details for parallelFor and parallelReduce within RcppParallel. This was harder than I expected because we allowed classes of the form:

struct InnerProduct : public RcppParallel::Worker {
  void join(const InnerProduct& rhs) { ... }
};

And so, we have no knowledge of the actual user-defined class or its type from within the RcppParallel worker. Hence, we need to take the template type passed to tbbParallelReduce(), use that to construct a whole bunch of type-erased methods within a non-template class, and then pass that blob down. Gross.

I'm hoping that, sometime after R 4.5.0 is released, we'll be able to delete this "glue" and have a more sane interface, but we'll see.

In any event, after doing all of this and then reinstalling gaston from sources, I see:

$ nm gaston/libs/gaston.so | grep tbb | grep U | c++filt
                 U RcppParallel::tbbParallelFor(unsigned long, unsigned long, RcppParallel::Worker&, unsigned long, int)
                 U RcppParallel::tbbParallelReduceImpl(unsigned long, unsigned long, RcppParallel::ReducerWrapper&, unsigned long, int)

and so we have now successfully hidden the TBB internals from gaston.

[kevinushey]

RcppParallel 5.1.10 built from this branch is now on CRAN; I'm going to take care of merging this (and handling merge conflicts) soon.