Review safety in low-level code

[Bromeon]

The current low-level implementation has some rough edges, such as unnecessary unsafe or possible UB. This issue tracks several code locations, in which robustness could be improved. A lot of those are marked with TODO or FIXME in code.

Soundness:

• Global access to everything uses static mut
• Global string registry (returning *const i8) hands out &mut at will
  • This was removed a long time ago, as I found out we don't need to retain the memory on our side.
• as_storage() not only hands out &mut, but also leaves them unbounded

Test coverage:

• Ptrcalls and pointer conversions
• Varcalls and variant conversions
• Reference-counter increments and decrements across several types
• Deref and cast between smart pointers/references to Godot objects
• Memory leak checks (mem::forget, missing dec_ref, etc.)

[Bromeon]

Updated several of the above tasks. Will spend some effort over the coming days to look into the rest and then close the issue.

[Bromeon]

With #546 and #581 merged, the last points in the above list should be addressed.