bindings from `converting` annotations can have the wrong scope

[mflatt]

Fails with an error about x defined multiple times:

def x :: converting(fun (x :: String) :: Int: x.to_number()) = "1"

Incorrectly allowed, since x shouldn't be available at the end:

def z :: converting(fun (x :: String) :: Int: x.to_number()) = "1"
x