UART and TWIM: Implement large buffer splitting and check for readonly buffers in flash

[simonsso]

Same type of changes as done for SPI.

I have not split up the dma transfer for: pub fn write_then_read()
it is not clear to me what assumptions can be made about read and write - for instance if a write and read always completes in sync or if all writes shall be done before data starts to be revived.

There have been no attempts to verify this change -- other than it is almost the same as I did for SPI.

[simonsso]

@jamesmunns / @hannobraun / @Yatekii Should we try to do something about this change?

This patch includes the fail-if-DMA is not in RAM check and replaces the 255 limit on easy DMA by splitting it up in several if there is a limit.

[hannobraun]

Sorry, I don't have time to take a closer look right now. I've added it to my list.

[Yatekii]

@simonsso I tried to speed up discussions about the implicit copy and batching.
We did also think about possible compiletime solutions.

Maybe it should be merged for now and adapted later on (even tho I think we'll never adapt it then :P).
If @jamesmunns and @hannobraun agree, I am ok with merging this for now if you fix the minor issue with bondary checks pointed out.

What do you think about this @jamesmunns @hannobraun? Appart from the ongoing issue about implicity, this seems to be sound otherwise (except for the minor issue pointed out by @somemetricprefix which can be fixed easily.).

[simonsso]

Close in favor of #74 and #75