Work around gcc 7 ICE

Current g++ 7 snapshot fails to compile pybind under -std=c++17 with:

```
$ make
[  3%] Building CXX object tests/CMakeFiles/pybind11_tests.dir/pybind11_tests.cpp.o
In file included from /home/jagerman/src/pybind11/tests/pybind11_tests.h:2:0,
                 from /home/jagerman/src/pybind11/tests/pybind11_tests.cpp:10:
/home/jagerman/src/pybind11/include/pybind11/pybind11.h: In instantiation of 'pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...)::<lambda(pybind11::detail::function_record*, pybind11::handle, pybind11::handle, pybind11::handle)> [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]':
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:120:22:   required from 'struct pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(struct pybind11::detail::function_record*, class pybind11::handle, class pybind11::handle, class pybind11::handle)>'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:120:19:   required from 'void pybind11::cpp_function::initialize(Func&&, Return (*)(Args ...), const Extra& ...) [with Func = pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]::<lambda(ConstructorStats*)>; Return = int; Args = {ConstructorStats*}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:62:9:   required from 'pybind11::cpp_function::cpp_function(Return (Class::*)(Arg ...), const Extra& ...) [with Return = int; Class = ConstructorStats; Arg = {}; Extra = {pybind11::name, pybind11::is_method, pybind11::sibling}]'
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:984:22:   required from 'pybind11::class_<type_, options>& pybind11::class_<type_, options>::def(const char*, Func&&, const Extra& ...) [with Func = int (ConstructorStats::*)(); Extra = {}; type_ = ConstructorStats; options = {}]'
/home/jagerman/src/pybind11/tests/pybind11_tests.cpp:24:47:   required from here
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:147:9: sorry, unimplemented: unexpected AST of kind cleanup_stmt
         };
         ^
/home/jagerman/src/pybind11/include/pybind11/pybind11.h:147:9: internal compiler error: in potential_constant_expression_1, at cp/constexpr.c:5593
0x84c52a potential_constant_expression_1
	../../src/gcc/cp/constexpr.c:5593
0x84c3c0 potential_constant_expression_1
	../../src/gcc/cp/constexpr.c:5154
0x645511 finish_function(int)
	../../src/gcc/cp/decl.c:15527
0x66e80b instantiate_decl(tree_node*, int, bool)
	../../src/gcc/cp/pt.c:22558
0x6b61e2 instantiate_class_template_1
	../../src/gcc/cp/pt.c:10444
0x6b61e2 instantiate_class_template(tree_node*)
	../../src/gcc/cp/pt.c:10514
0x75a676 complete_type(tree_node*)
	../../src/gcc/cp/typeck.c:133
0x67d5a4 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool, bool)
	../../src/gcc/cp/pt.c:17516
0x67ca19 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool, bool)
	../../src/gcc/cp/pt.c:16655
0x672cce tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:16140
0x6713dc tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15408
0x671915 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15394
0x671fc0 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15618
0x66e97f tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
	../../src/gcc/cp/pt.c:15379
0x66e97f instantiate_decl(tree_node*, int, bool)
	../../src/gcc/cp/pt.c:22536
0x6ba0cb instantiate_pending_templates(int)
	../../src/gcc/cp/pt.c:22653
0x6fd7f8 c_parse_final_cleanups()
	../../src/gcc/cp/decl2.c:4512
```

which looks a lot like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77545.

The error seems to be that it gets confused about the `std::tuple<...>
value` in argument_loader: it is apparently not being initialized
properly.  Adding a default constructor with an explicit
default-initialization of `value` works around the problem.

Author: jagerman

include/pybind11/cast.h

@@ -1220,6 +1220,8 @@ class argument_loader {
     using indices = make_index_sequence<sizeof...(Args)>;
 
 public:
+    argument_loader() : value() {} // Helps gcc-7 properly initialize value
+
     static constexpr auto has_kwargs = std::is_same<itypes, type_list<args, kwargs>>::value;
     static constexpr auto has_args = has_kwargs || std::is_same<itypes, type_list<args>>::value;