arg: Avoid data corruption when marshalling unsupported flat array

Currently, when a function is supposed to receive a flat array (for
example, regress_test_array_struct_in_full() taking RegressTestStructA*)
GJS will instead construct a pointer array (RegressTestStructA** in the
above example) and try to pass it to the function. This leads to garbage
data being passed to the function and possibly crashes.

Anticipate this situation and throw an exception instead.

Related to #44.