API for dropping Zaplib buffers

[janpaul123]

Something like zaplib.drop(buffer) to delete the buffer and make it unusable (just like when you pass a mutable buffer to Rust when passing ownership).

Currently you cannot control when immutable buffers are deallocated (it depends on garbage collection), so this gives you some control over it; thereby freeing memory that you don't use. With mutable buffers the workaround is to transfer it to Rust using callRustSync, and then immediately dropping it there.