Function dispatch_request 

pub fn dispatch_request<H: HostIo>(
    host: &mut H,
    req: RequestType,
    input: &[u8],
) -> Result<(Vec<u8>, Vec<u8>, u64), H::Error>

Dispatch a host I/O request to the appropriate HostIo method.

Takes a RequestType and input bytes and returns (result, extra_data, gas_cost).