Require all SPI and Serial word types to be Copy

[GrantM11235]

The word type should always be a builtin integer (u8, u16, etc) or a newtype to represent an integer with an unusual size, which will almost certainly be Copy.

I also removed the 'static bound from the spi transactional stuff because it appears to be unnecessarily now.

[rust-highfive]

r? @ryankurte

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[eldruin]

Requiring Copy for word types seems fine to me.
I am not sure what the reason for the 'static bound was so I will leave that to @ryankurte

[Dirbaio]

Perhaps we want to require 'static for all words? I can't imagine a legit reason for it not to be static

[ryankurte]

i had to go digging [1, 2], it appears the justification was to separate the lifetime of the type W from that of the buffers &'a [W], i can't imagine i would have added it if unnecessary, but, if the compiler can infer the lifetime of the word type now perhaps that is no longer required.

Copy sgtm too. there aren't a lot of reasons for W not to be static, but, it's also seemingly an unnecessary bound so, provided it is demonstrated to still work on both sides i'd be okay with removing the 'static.

[GrantM11235]

I decided to leave the 'static bound unchanged for now because I don't fully understand it, and it seems to be unrelated to this PR.

[eldruin]

Looks good to me, thanks!
bors r+

[bors]

Build succeeded:

• ci-linux (1.46.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.46.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt