Maybe you would like UFCS: https://en.wikipedia.org/wiki/Uniform_Function_Call_Syntax

e.g. Nim has it, and it lets you do this:

  proc add(self: MyList, e: int) =
    # [insert append code here]

  mylist.add(1) # equivalent to
  add(mylist, 1)

Though I'm not sure how well it would work with C, considering it lacks function overloading.