Redesign argument storing

* Use state objects instead of tons of locals in the main
  function.
* Use actions for storing arguments into result table.
  Actions are now called at the end of each invocation,
  with result table, target index, arguments and overwrite flag as
  arguments.
* Remove command actions.
* Improve error messages, refer to options by the last used alias
  instead of the main name.

TODO:

* Improve error messages further ("argument 'foo' is required"
  -> "missing argument 'foo'", etc.).
* Add actions for positional arguments.
* Add actions for commands (should be called with final results
  after parsing is over, in "innermost first" order).
* Allow referring to built-in actions by strings a-la Python
  (e.g. action = "store_false").
* Allow setting initial value to be stored at target index
  for each option (perhaps use default value for that).
* Add more tests, particularly for actions.

spec/default_spec.lua

@@ -55,7 +55,7 @@ describe("tests related to default values", function()
          assert.same({output = "a.out"}, args)
          args = parser:parse{"--output", "foo.txt"}
          assert.same({output = "foo.txt"}, args)
-         assert.has_error(function() parser:parse{"-o"} end, "too few arguments")
+         assert.has_error(function() parser:parse{"-o"} end, "option '-o' requires an argument")
       end)
 
       it("handles option with default value for multi-argument option correctly", function()