Revise design of `conversion_dispatch` #443

Job	Run time
Run BVT with MSVC / bvt-msvc	3m 22s
Run BVT with GCC / bvt-gcc	2m 45s
Run BVT with AppleClang / bvt-appleclang	1m 58s
Run BVT with Clang / bvt-clang	5m 57s
Run BVT with NVHPC / bvt-nvhpc	4m 19s
Generate report / bvt-report	11s
	18m 32s

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