Add `TUPLE_UNION` and `TUPLE_MERGE`; Add 'vararg' functions #2704

Job	Run time
cargo-deny (advisories)	23s
Build and Test (ubuntu-20.04)	6m 42s
Create conformance report for `push` and `pull_request` events	6m 23s
cargo-deny (bans licenses sources)	23s
Build and Test (windows-latest)	8m 7s
Build and Test (macos-latest)	5m 9s
Create comparison report for `pull_request` event	0s
	27m 7s

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