Allow transparent parameters at any staging level

[nicolasstucki]

Inline parameters are currently only allowed at level 0 and 1. But they should be available at any level.

transparent def foo(transparent x: Int): Int = ~fooImpl(x, '(x), '( '(x) ), '( '( '(x) ) ))
def fooImpl(a: Int, b: Expr[Int], c: Expr[Expr[Int]], d: Expr[Expr[Expr[Int]]]): Expr[Int] = ???

This makes inline values behave the same way as transparent members do.

In general, the PCP rule should be updated to say that inline values are not free variables of the quote.