[WIP] Improve windows support

[auterium]

This PR includes:

• [x] Basic logging support for Windows. Regular, PG-based, logging support crashes under Windows at runtime, so using eprintln!() we can at least avoid crashes and write to PG log.
• [ ] Side effects of logs under Windows: notify back to the client any messages that should be sent back
• [x] Soundess in &[T] support: only execute unsafe code inside unsafe blocks; safe code moved out of unsafe blocks
• [x] Document how to use correct rustflags for Windows builds (pointing to postgres.lib)