fix ireference usage in structs

[stevenbrix]

when someone tries to include a struct with reference types, the code gen breaks for a few reasons:

1. in the constructor of the projected struct, we were using IReference<T> instead of T?
2. in the abi conversion code, we weren't converting between the ABI and swift types appropriately

Changes

• for (1) above, use the write_type api
• for (2) above, check if the member type is an ireference and appropriately set the value

Tests

added test cases

Fixes WIN-450

[darinf]

Awesome, thank you!