Use named arguments for {,u}int_impls macro

[Noratrieb]

This makes it way easier to understand.

r? @scottmcm

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[est31]

I wonder if it makes sense to distinguish here between "used in docs" vs "not used in docs", say via a doc_ prefix, but looking at the code, it seems that basically every parameter is only used in docs except for ActualT...

[scottmcm]

it seems that basically every parameter is only used in docs

Hmm, I suppose there's something fundamental here: the only reason that these have to be macro parameters at all is to use them lexically in docs.

Whenever they're used by actual code, they could have just been private associated types/constants instead. (For example, it could use Self::UnsignedType/Self::SignedType instead of $UnsignedT/$SignedT, and just define those in the impl block, rather than passing them as parameters.)

[scottmcm]

But regardless, this PR is still good-to-go on its own as a nice improvement

@bors r+ rollup

(It's making me think that maybe BITS_MINUS_ONE being an expr isn't useful, for example -- if it's not a literal then it could just use ($BITS-1) if it needed it. Similarly for BITS, which is always sizeof(Self)*8 if it being an expr is fine. But none of those things are this PR's problem.)

[bors]

📌 Commit eb5d82b has been approved by scottmcm

It is now in the queue for this repository.