Added a &malloc function for FFI

[ekgame]

This PR implemets a &malloc function to allocate memory that can then be passed to an FFI function as a pointer to populate it with data. The data can then be read back as bytes with &memcpy and freed with &memfree.

[ndren]

If the buffer is allocated with

let mut buffer = vec![0u8; size];

and deallocated with

unsafe { drop(Box::from_raw(ptr.get_mut())) };

is that valid? I'm not at all familiar with the exact semantics but I don't know if casting a Vec to a Box like that is promised to work and I can't find such a promise from the documentation. (I could be entirely wrong though, the code does work)

[kaikalii]

If that's a concern, maybe allocate with Box::new_uninit_slice or Box::new_zeroed_slice instead. Then just get the pointer with Box::leak.

[ekgame]

If that's a concern, maybe allocate with Box::new_uninit_slice or Box::new_zeroed_slice instead. Then just get the pointer with Box::leak.

new_zeroed_slice is not stable yet (rust-lang/rust#129396), so I changed it to use new_uninit_slice.