Refactor parser

Major refactor of the parsing code organisation to improve encapsulation
and not modify the input arguments. The returned result no longer has
pointers into the original option specification.

test/options.cpp

@@ -154,7 +154,7 @@ TEST_CASE("All positional", "[positional]")
 
   auto result = options.parse(argc, argv);
 
-  REQUIRE(argc == 1);
+  CHECK(result.unmatched().size() == 0);
   REQUIRE(positional.size() == 3);
 
   CHECK(positional[0] == "a");
@@ -182,7 +182,7 @@ TEST_CASE("Some positional explicit", "[positional]")
 
   auto result = options.parse(argc, argv);
 
-  CHECK(argc == 1);
+  CHECK(result.unmatched().size() == 0);
   CHECK(result.count("output"));
   CHECK(result["input"].as<std::string>() == "b");
   CHECK(result["output"].as<std::string>() == "a");
@@ -209,11 +209,10 @@ TEST_CASE("No positional with extras", "[positional]")
   auto old_argv = argv;
   auto old_argc = argc;
 
-  options.parse(argc, argv);
+  auto result = options.parse(argc, argv);
 
-  REQUIRE(argc == old_argc - 1);
-  CHECK(argv[0] == std::string("extras"));
-  CHECK(argv[1] == std::string("a"));
+  auto& unmatched = result.unmatched();
+  CHECK((unmatched == std::vector<std::string>{"a", "b", "c", "d"}));
 }
 
 TEST_CASE("Positional not valid", "[positional]") {
@@ -643,9 +642,9 @@ TEST_CASE("Unrecognised options", "[options]") {
 
   SECTION("After allowing unrecognised options") {
     options.allow_unrecognised_options();
-    CHECK_NOTHROW(options.parse(argc, argv));
-    REQUIRE(argc == 3);
-    CHECK_THAT(argv[1], Catch::Equals("--unknown"));
+    auto result = options.parse(argc, argv);
+    auto& unmatched = result.unmatched();
+    CHECK((unmatched == std::vector<std::string>{"--unknown", "--another_unknown"}));
   }
 }