This Adds a bunch of tests for passing and returning structs
of various sizes to C. It fixes the struct return rules on unix,
and on windows for structs of size > 8 bytes. Struct passing
on unix for structs under a certain size appears to still be broken.
A struct (inc. tuple struct) can be annotated with #[packed], so that there
is no padding between its elements, like GCC's `__attribute__((packed))`.
Closes#1704
