Stricter memory safety when `omni_alloc` returns nil

[vitreo12]

Right now, these checks are only done in the constructor of structs, but the nil fields would still be accessible elsewhere in the init block, probably causing crashes. try / raise / except cannot be used as they require the initialization of Nim's runtime to work.

• Enforce safe access to any struct field in the init block under the hood. This needs for every access to a struct field to first check if the struct is nil. This is the better real-time safe option, but it requires some thinking, especially as this can not only happen on assignment, e.g. struct.field = something, but also if passing the struct.field around.

[vitreo12]

Check the memory-safety branch, which fixes this!

[vitreo12]

Remaining issues:

1) What about if/while/loop/for statements? Right now the check is only added before them, but not in the body (which can still potentially do bad things).... DONE

2) What about dynamic data allocation? Right now it would still return a nil value if it fails to allocate, which is problematic when chaining things: data[0][0]. The first [0] can trigger failure in allocation, returning a nil which can't be checked, crashing

[vitreo12]

Final implementation involved wrapping setjmp and longjmp manually, and it's working flawlessy.

branch: memory-safety-custom-setjmp