Exercise: Reverse Polish Calculator

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• Implement a reverse polish calculator.

Examples:

go run rpc.go "2 7 + =" 
9

go run rpc.go "2.1 7.2 - =" 
-5.1

go run rpc.go "7 2 / =" 
3.5

go run rpc.go "2 3 5 + * ="
16