Add operator for calling a function with the preceeding object as first argument.

[lrhn]

Example syntax:
  object->function(arg2, arg3)
which is syntactic sugar for
  function(object, arg2, arg3)
but with the same precedence as '.'.

This allows chaining a function call that isn't a method.
Currently calling a function on the result of a chain of method calls needs to wrap everything with the function. E.g.:

  func2(func1(object.m1().m2().m3()).m4().m5()).m6()

could become the much more readable:

  object.m1().m2().m3()->func1().m4().m5()->func2().m6()

(This is primarily a work-around for not having extension methods.)

[lrhn]

Added Duplicate label.
Marked as being merged into #16900.