Beta-reduce directly applied PolymorphicFunction

[nicolasstucki]

Beta-reduce directly applied PolymorphicFunction such as

([Z] => (arg: Z) => { def a: Z = arg; a }).apply[Int](2)

into

type Z = Int
val arg = 2
def a: Z = arg
a

Apply this beta reduction in the BetaReduce phase and Expr.betaReduce. Also, refactor the beta-reduce logic to avoid code duplication.

Fixes #15968