Adds support for passing and returning structs as buffers to FFI. This does not implement fastapi support for structs. Needed for certain system APIs such as AppKit on macOS.
This commit removes three unstable Deno APIs:
- "Deno.spawn()"
- "Deno.spawnSync()"
- "Deno.spawnChild()"
These APIs were replaced by a unified "Deno.Command" API.
Refactors the `Deno.Command` class to not handle any state, but only being an intermediary to calling its methods, and as such any methods and properties besides `output`, `outputSync` & `spawn` have been removed. Interracting with a `spawn`ed subprocess now works by using the methods and properties on the returned class of the `spawn` method.
