SPI `TransferInPlace` operation is unsound.

The implementation for the TransferInPlace operation violates the aliasing rule by creating an immutable reference from a mutable one, but keeping both.

linux-embedded-hal/src/spi.rs

Lines 216 to 222 in 1ffb268

    
           SpiOperation::TransferInPlace(buf) => { 
        
               let tx = unsafe { 
        
                   let p = buf.as_ptr(); 
        
                   std::slice::from_raw_parts(p, buf.len()) 
        
               }; 
        
               transfers.push(SpidevTransfer::read_write(tx, buf)); 
        
           }

There needs to be a corresponding method for SpidevTransfer, e.g. read_write_in_place.

I definitely concur with this being UB! Would be great to have this fixed, we've notified @sirhcel, who will be taking a peak when possible :)

This could be fixed with #123, but needs a release of spidev first, to implement the required read_write_in_place function.
(with rust-embedded/rust-spidev#49)

	SpiOperation::TransferInPlace(buf) => {
	let tx = unsafe {
	let p = buf.as_ptr();
	std::slice::from_raw_parts(p, buf.len())
	};
	transfers.push(SpidevTransfer::read_write(tx, buf));
	}