Add `_SAVE` & `_SAVE_AS` for unnamed/named captures about parsertoy

[xparq]

Just FTR:

CASE("CAPTURE: named") {
    Parser p(_{
	    _{_OPT, "_WHITESPACES"},
	    _{_SAVE_AS, "id", "_ID"},
	    _{_OPT, "_WHITESPACES"},
	    "=",
	    _{_OPT, "_WHITESPACES"},
	    _{_SAVE_AS, "val", "_DIGITS"}
    }); p.syntax.DUMP();

    CHECK(p.parse("  capture_this = 1  "));
	    CHECK(p.unnamed_captures.empty());
	    CHECK(p.named_captures.size() == 2);
	    CHECK(p["id"] == "capture_this");
	    CHECK(p["val"] == "1");
    CHECK(p.parse("  or_this = 22 // Wow, fake comments! ;)"));
	    CHECK(p.unnamed_captures.empty());
	    CHECK(p.named_captures.size() == 2);
	    CHECK(p["id"] == "or_this");
	    CHECK(p["val"] == "22");

    CHECK(!p.parse("not this = one"));
}

CASE("CAPTURE: nested") {
    RULE code = _{_MANY, _{_OR, "_ID", "=", "_DIGITS", ";", "_WHITESPACES"} };
    RULE block_in = _{"<", _{_SAVE_AS, "inner", code}, ">"};
    RULE block_out = _{"<", _{_SAVE_AS, "outer",
			    _{ _{_OPT, code}, _{_OPT, block_in}, _{_OPT, code} },
		    }, ">"};

    Parser p(block_out); p.syntax.DUMP();

    CHECK(p.parse("<outer <inner> block>"));
    CHECK(p["inner"] == "inner");
    CHECK(p["outer"] == "outer <inner> block");

    CHECK(p.parse("<outer < x = 1; y = 2; > block>"));
    CHECK(p["inner"] == " x = 1; y = 2; "); //! Mind the spaces...
    CHECK(p["inner"] !=  "x = 1; y = 2;" ); //! Mind the spaces...
    CHECK(p["outer"] == "outer < x = 1; y = 2; > block");
}