#2070: variant: replace SafeUnion with std::variant

[lifflander]

Fixes #2070

[github-actions]

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PR tests (clang-9, ubuntu, mpich)

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PR tests (gcc-9, ubuntu, mpich, zoltan, json schema test)

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PR tests (gcc-10, ubuntu, openmpi, no LB)

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PR tests (intel icpc, ubuntu, mpich)

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PR tests (nvidia cuda 11.2, gcc-9, ubuntu, mpich)

Build for 94c8e86 (2023-09-12 09:14:23 UTC)

/vt/src/vt/pipe/pipe_manager.impl.h(133): warning: missing return statement at end of non-void function "vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&vt::vrt::collection::lb::GreedyLB::collectHandler, Target=vt::objgroup::proxy::ProxyElm<vt::vrt::collection::lb::GreedyLB>]"
          detected during:
            instantiation of "auto vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&vt::vrt::collection::lb::GreedyLB::collectHandler, Target=vt::objgroup::proxy::ProxyElm<vt::vrt::collection::lb::GreedyLB>]" 
/vt/src/vt/objgroup/proxy/proxy_objgroup.impl.h(154): here
            instantiation of "vt::objgroup::proxy::Proxy<ObjT>::PendingSendType vt::objgroup::proxy::Proxy<ObjT>::reduce<f,Op,Target,Args...>(Target, Args &&...) const [with ObjT=vt::vrt::collection::lb::GreedyLB, f=&vt::vrt::collection::lb::GreedyLB::collectHandler, Op=vt::collective::PlusOp, Target=vt::objgroup::proxy::ProxyElm<vt::vrt::collection::lb::GreedyLB>, Args=<vt::vrt::collection::lb::GreedyPayload>]" 
/vt/src/vt/vrt/collection/balance/greedylb/greedylb.cc(222): here

/vt/src/vt/pipe/pipe_manager.impl.h(133): warning: missing return statement at end of non-void function "vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&MyObj::handler, Target=vt::objgroup::proxy::ProxyElm<MyObj>]"
          detected during instantiation of "auto vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&MyObj::handler, Target=vt::objgroup::proxy::ProxyElm<MyObj>]" 
/vt/examples/callback/callback.cc(147): here

/vt/src/vt/pipe/pipe_manager.impl.h(133): warning: missing return statement at end of non-void function "vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&colHan, Target=vt::vrt::collection::VrtElmProxy<MyCol, vt::Index1D>]"
          detected during instantiation of "auto vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&colHan, Target=vt::vrt::collection::VrtElmProxy<MyCol, vt::Index1D>]" 
/vt/examples/callback/callback.cc(153): here

/vt/src/vt/pipe/pipe_manager.impl.h(133): warning: missing return statement at end of non-void function "vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&MyObj::handler, Target=vt::objgroup::proxy::ProxyElm<MyObj>]"
          detected during instantiation of "auto vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&MyObj::handler, Target=vt::objgroup::proxy::ProxyElm<MyObj>]" 
/vt/examples/callback/callback.cc(147): here

/vt/src/vt/pipe/pipe_manager.impl.h(133): warning: missing return statement at end of non-void function "vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&colHan, Target=vt::vrt::collection::VrtElmProxy<MyCol, vt::Index1D>]"
          detected during instantiation of "auto vt::pipe::PipeManager::makeSend<f,Target>(Target) [with f=&colHan, Target=vt::vrt::collection::VrtElmProxy<MyCol, vt::Index1D>]" 
/vt/examples/callback/callback.cc(153%0D%0A%0D%0A%0D%0A ==> And there is more. Read log. <==

Build log

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[nlslatt]

I have exactly the same questions/comments as Arek.

[thearusable]

@nlslatt - I addressed the comments we made on this PR. I think this can be merge now.

[nlslatt]

Looks good