TY-1759 move serialized bytes allocation from dart to rust

[janpetschexain]

References

• TY-1759

Summary

• add an allocator for CBytes in rust and use it in the dart wrapper: we don't have to differentiate between owned/borrowed for the Bytes in dart, destructor always runs in rust; but this can potentially panic, which requires more complex try-catches in dart.
• rearrange the rust ffi docs a bit: the blob is becomming quite large, hence i exposed the first layer of modules in the documentation to structure it in a more fine grained way

also the ExternError is getting more and more annoying (cumbersome message freeing, unnecessary *const/*mut casts, i32 limitation for the code, no impl From<Infallible> for ExternError, etc). depending on how we want to unify the c error handling with the wasm error handling after #51, we should think about refactoring this as well.

[acrrd]

My idea when I wrote the comment was to move only the allocation and not the copy. So just bytes_new(size: 32) that behaves like a Vec::with_capacity(size).leak(). This will avoid the double allocation that you mention. Will not change much but we have allocation and free all in rust and we don't have two version of free.

[janpetschexain]

My idea when I wrote the comment was to move only the allocation and not the copy. So just bytes_new(size: 32) that behaves like a Vec::with_capacity(size).leak(). This will avoid the double allocation that you mention. Will not change much but we have allocation and free all in rust and we don't have to version of free.

ah ok, i see what you meant, i'll update the pr accordingly.