Consistent naming of source, sender, payer

The use of the words "source", "sender", and "payer" is not consistent. They are used at (at least) 3 places:

• as Michelson instructions "SOURCE" and "SENDER"
• as command-line options to the run script client command (--payer and --source)
• as variable names in the code base (client and protocol).

In particular, using source to refer to the result of the SENDER instruction is particularly confusing given that there is a SOURCE instruction returning something else.

The following discussion from !1539 (closed) should be addressed:

• @rafoo_ started a discussion: (+1 comment)

  In my opinion, we should change the command-line option names to stick to the names of the Michelson instructions. We should also change the names of the variables in the OCaml code.

  It is small but in the protocol...

  By the way, the client code is protocol-specific but it is not "in the protocol". In particular it is not part of the code that is subject to the amendment voting procedure.