Add support for expressions in operation definition

[dumconstantin]

Example:

const prodA: producer = ({
  a = observe.foo || observe.bar,
  b = observe.bam || 123,
  c = arg.b === 123,
  d = arg.c && arg.c + 10,
  e = arg.d > 100,
  f  = (observe.baz).keys(),
  g =  observe.a + observe.b,
  h = (observe.x).substr(10),
  i = shortId() // this is not intuitive - to postpone the execution until the producer executes
}) => {}

This allows providing default values and making simple operations.