Add `AsyncEnumerable.Merge` APIs #703

Job	Run time
Test (Release, Pool_Enabled, devMode)	55m 21s
Test (Release, Pool_Enabled, userMode)	53m 37s
Test (Release, Pool_Disabled, devMode)	53m 57s
Test (Release, Pool_Disabled, userMode)	51m 10s
Test (Release_NoProgress, Pool_Enabled, devMode)	41m 0s
Test (Release_NoProgress, Pool_Enabled, userMode)	35m 46s
Test (Release_NoProgress, Pool_Disabled, devMode)	37m 44s
Test (Release_NoProgress, Pool_Disabled, userMode)	33m 51s
Test (Debug, Pool_Disabled, devMode)	55m 0s
Test (Debug, Pool_Disabled, userMode)	54m 43s
Test (Debug_NoProgress, Pool_Disabled, devMode)	39m 16s
Test (Debug_NoProgress, Pool_Disabled, userMode)	38m 48s
	9h 10m 13s

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