Reduce `unsafe` from the function declarations to smallest possible `unsafe` blocks

[kud1ing]

Should we hide unsafe inside the functions or is more honest/beneficial to leak unsafe in the function declarations as it is?

Discussion from the perspective of keep leaking unsafeness:

• Pro:
  • what Rucaja does is possibly unsafe
  • the JNI is low-level and not well documented
  • but we try hard to use JNI correctly and thus everything we do should be safe, unless it is a bug
• Con:
  • forces users to deal with unsafeness

[fpoli]

As a general rule I would prefer to expose the unsafe whenever Rucaja is possibly unsafe. That is, a crash / undefined behavior that happens while using a safe Rucaja function should be a bug of Rucaja.

[treyzania]

@fpoli That's true. Like how you can do magic things like Vec::from_raw_parts. But if we can make some Rucaja functionality definitely safe for some subset of behavior then it absolutely makes sense to wrap it with safe, idiomatic Rust.

[fpoli]

I agree

[kud1ing]

I made up my mind and i agree that we should reduce the scope of unsafeness as much as possible.

We should prevent invalid usage of JNI, as we currently never store raw null-pointers. Crashes and unexpected null-pointer-exceptions in Java-land are bugs.

[kud1ing]

Fixed by https://github.com/kud1ing/rucaja/commit/55c32ab2ca252912d2ffecc18ce44be7781e0e5f