refactor(ir): impure values are never dereferencible

[kszucs]

All impure values passed to the API methods are treated as separate impure invocations:

impure = ibis.random()
t1 = t.mutate(y=impure)
t2 = t1.mutate(z=impure.cast("string"))

now produce

r0 := UnboundTable: unbound_table_0
  a int64
  b string

r1 := Project[r0]
  a: r0.a
  b: r0.b
  y: RandomScalar()

Project[r1]
  a: r1.a
  b: r1.b
  y: r1.y
  z: Cast(RandomScalar(), to=string)