e.g.
```
def m(a,(b,c),d); p [a,b,c,d]; end
m(1,[2,3],4)  # => [1,2,3,4]
```

mruby limitation:
Destructured arguments (`b` and `c` in above example) cannot be accessed
from the default expression of optional arguments and keyword arguments,
since actual assignment is done after the evaluation of those default
expressions. Thus:

```
def f(a,(b,c),d=b)
  p [a,b,c,d]
end
f(1,[2,3])
```

raises `NoMethodError` for `b` in mruby.