Event buffer in SendEventBuffer is not allocated with appropriate alignment

The buffer for storing the variable-size [`Events`](https://github.com/RustAudio/vst-rs/blob/fd8e173b9a7db9a8f924bbed14de99d3210188c1/src/api.rs#L410-L422) structure in `SendEventBuffer` is [allocated as a `Vec<u8>`](https://github.com/RustAudio/vst-rs/blob/fd8e173b9a7db9a8f924bbed14de99d3210188c1/src/buffer.rs#L396), which does not have the alignment required for `Events`.