Add DMA support to USART API #20
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I'm working on some changes that require `Transfer::new` to be opened up to the rest of the crate, so I think it makes sense to not require weird PAC types when calling it.
The problem with these constructors is that they're not flexible enough. In general, peripheral APIs might want to present DMA transfers to the user in a different way than they work in hardware. For example, the AES peripheral supports only transfers of 32-bit words, but might want to present an API that works with byte slices (I'm currently working on this). Another example would be SPI, which might want to hide two simultaneous DMA transfers (for read and write) behind a single transfer struct. This commit doesn't make these use cases possible yet, as some modifications to the unsafe constructor are still required. But it paves the way, by exposing that constructor to the peripheral APIs. Please note that none of this exposes any unsafety to the users. They will still use the same (safe) peripheral APIs to access DMA.
The new trait, `Buffer`, provides everything the DMA code needs from the buffer, without requiring implementers to be able to return a slice. This makes the trait more flexible, as it can be implemented for a larger variety of types, without requiring any complex transmuting.
The type parameter names were wrong, as they were in a different order than was used everywhere else in the code. Please note that this didn't cause any incorrect behavior, as just the names were wrong and nothing else.
This should be more natural than operating with `u32` words.
This doesn't add DMA support to USART yet, as that still requires special DMA APIs in the USART module.
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Also contains some small cleanups of the
serialmodule.This pull request is based on #19. I recommend reviewing/merging that pull request first.
cc @lthiery