[BUG] `for` loop don't allow several assignments in `next`

[realgdman]

for loop (and other loops) don't accept several assignments inside next

Reproduce Code
https://cpp2.godbolt.org/z/aqYqr63z5

main: () = {
	(copy a := 0, copy b := 0)
	//for "abc" next a++ do (i) { //ok
	for "abc" next { a++; b++; } do (i) { //error
	//for "abc' next a do (i) { //unused a
		std::cout << i << a << b << '\n';
	}
}

Version
latest (38aec57)

Command lines
cppfront/cppfront $1.cpp2 -p
clang++-15 -Icppfront/include $1.cpp -std=c++20 -o $1

Expected result
Some block syntax for that assignment or diagnostic

Actual result
Compilation error

Additional notes
In grammar only single expr allowed
//G next-clause:
//G 'next' assignment-expression

Additionally, logical-or-expression could be logical.
Which can lead to compiled, but unused logical expression
for "abc' next a do (i) { //unused a

[realgdman]

Temporary workaround was found

main: () = {
	(copy a := 0, copy b := 0, next :=:()={ a&$*++; b&$*++; })
	for "abc" next next() do (c) {
		std::cout << c << a << b << '\n';
	}	
}

First attempt

``` main: () = { a:=0; b:=0; l := :() = { a&$*++; b&$*++; }; for "abc" next l() do (c) { std::cout << c << a << b << '\n'; } } ```

Thanks for idea from here, https://github.com//issues/432 currently lambda cannot be declared inside `next`

[msadeqhe]

The example should be written like this:

main: () = {
	(copy a: = 0, copy b: = 0)
	for "abc" next (a++, b++) do (i) {
		std::cout << i << a << b << '\n';
	}
}

Cpp2 generates this:

auto main() -> int{
{
auto a = 0;
auto b = 0;

 { auto const& cpp2_range = "abc"; for ( auto const& i : cpp2_range )  { do {
  std::cout << i << a << b << '\n';
 } while (false); (++a, ++b); }}
}
}

[realgdman]

Well I should have guessed. Even this works.
for "abc" next ( a++, b++, std::cout << a, foo()) do (c)

[JohelEGP]

See #449 (comment). It may work by accident.