WIP: update to e-h 1.0.0-alpha.6

[ryankurte]

changes to SPI (rust-embedded/embedded-hal#287) mean forward compat for spi::Transfer and spi::Exec is no longer possible :-/

if we want to continue providing compatibility the easiest is probably to backport a transfer_split(write: &[u8], read: &[u8]) to 0.2.x.

cc. @eldruin @Dirbaio

[edit] solved below, thanks folks!

[Dirbaio]

You can copy from the write buffer to the read buffer, then do a single-buffer Transfer on the read buffer. The only case where this doesn't work is where the read buffer is smaller, which should be very, very rare.

[ryankurte]

ahh brilliant, not sure how i missed that, thanks!