Add DMA RX and RX/TX support to SPI

[Windfisch]

Implements #200

SPI is a bit different from most other DMA-supporting peripherals, since SPI in master mode cannot receive without transmitting simultaneously. After some discussion in the #rust-embedded irc channel, we came to the conclusion that split()ting the device (like with serial) does not make much sense, because sending and receiving are inherently tied to each other; that made it necessary to finish the implementation of the already-existing (but unused) RxTxDma struct from dma.rs and add required traits.

This PR does that and adds an RxTxDma implementation to the SPI module.

Because in slave mode, this might(?) be still possible to receive without transmitting, I left the RxDma implementations in, but I am yet unsure about this.

Please do tell me if you have any suggestions :).

[Windfisch]

An usage example can be found at https://github.com/Windfisch/bluepill_spidemo/blob/aa0bf2f6f24ed28cb3263e4e6f73a968538e5d70/src/main.rs#L50-L53

[Windfisch]

I added a release function that allows to undo with_tx_dma etc. Also, the location of setting the actual DMA bits was changed from SpiTxDma::start to with_tx_dma.

[burrbull]

What about add more type aliases?
Something like

pub type Spi1TxDma<REMAP, PINS> = SpiTxDma<SPI1, REMAP, PINS, dma::C3>;

[burrbull]

And do we really need SpiPayload wrapper?

[Windfisch]

nope, the SpiPayload wrapper was rather useless; I have removed it and added the type aliases you suggested

[burrbull]

bors r+

[bors]

Build succeeded:

• ci (stm32f100, stable)
• ci (stm32f101, stable)
• ci (stm32f103, stable)
• ci (stm32f105, stable)
• ci (stm32f107, stable)
• Rustfmt