C gen: make variants unboxed #3490

Job	Run time
Build	31s
Check Fir formatting	46s
Check Rust formatting	9s
Compiler unit tests	1m 0s
Check that generated files are up-to-date	1m 18s
Compiler golden tests	1m 7s
Check lints	24s
Interpreter unit tests	34s
C backend golden tests	1m 46s
Interpreter golden tests	1m 10s
Compiler parser tests	44s
Check Wasm build	43s
Formatter golden tests	1m 24s
	11m 36s

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