Allow inlining simple predicates in filters

[ReedOei]

It is often desirable to write something like:

type User is { name : string, isAdmin : bool }
transformer isAdmin(user : User) -> res : bool {
    copy(user.isAdmin) --> res
}
users --[ any such that isAdmin(_) ]--> var admins : list User

However, it is annoying to manually declare isAdmin, and it should be easy to automatically generate it, given something like:

users --[ any user such that user.isAdmin ]-> var admins : list User