Implement Drop for DMA transfer types

[thalesfragoso]

Right now the DMA API is unsound, the problem happens if you use a smart pointer (Box, etc) that deallocates on drop. In that case, the DMA will continue the write/read to the invalid region if the user drops the transfer type.

The Drop implementation should stop the DMA. For more information and example see the embedonomicon.

CC @korken89

[korken89]

This will be fixed in #230 (cont. of #165)