Add `if-then-else` statement support #159

Job	Run time
Prepare	4s
Docs - stable toolchain (recent)	1m 3s
Check WASM	30s
Test - stable toolchain	3m 44s
Test - nightly toolchain	4m 4s
Docs - nightly toolchain (recent)	3m 30s
Lint - nightly toolchain (recent)	53s
Format - nightly toolchain	24s
Test - MSRV	4m 8s
Check Fuzz	3m 0s
	21m 20s

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