spi/blocking: Add word methods and default impls that use them

[GrantM11235]

• Rename all methods that deal with slices to include "slice".
• Add read_word, write_word, and transfer_word methods.
• Add default implementations for slice methods based on word methods.

This will also allow us to add new methods in the future with default implementations as a non-breaking change.

[rust-highfive]

r? @therealprof

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[Dirbaio]

I'm not convinced the _word variants are useful.

• They can't be implemented with DMA, so I would not add them to the async traits, but this makes async+blocking inconsistent.
• I'm not sure it has a performance advantage. If you call write with a slice with len=1 the compiler can inline write and then simplify all the code baking in the assumption that len=1, for example eliminating the loop. This would give final compiled code similar to what write_word would've.

This will also allow us to add new methods in the future with default implementations as a non-breaking change.

Why is that not the case already, with the current slice-based methods?

[eldruin]

I share @Dirbaio's analysis. I would rather not increase the API surface if there is no very good reason for it. Could you elaborate what that would be?

[GrantM11235]

As discussed in the meeting, I find the arguments against these methods to be convincing.