Optimizing generator function

[kos59125]

Using ReflectedDefinition attribute and traversing generator functions.

[<ReflectedDefinition>]
let generator = random {
   // random process
}

// optimize : Expr<GeneratorFunction<'a>> -> GeneratorFunction<'a>
let optimizedGenerator = optimize <@ generator @>

[kos59125]

Put in the optimization branch. Only placeholder, no optimization is implemented.