chore: Rename `nat_cast`/`int_cast`/`rat_cast` to `natCast`/`intCast`/`ratCast`

[YaelDillies]

Now that I am defining NNRat.cast, I want a definitive answer to this naming issue. Plenty of lemmas in mathlib already use natCast/intCast/ratCast over nat_cast/int_cast/rat_cast, and this matches with the general expectation that underscore-separated name parts correspond to a single declaration.

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[eric-wieser]

Let's solicit a few more votes on https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/naming.20conventions.3A.20natCast.20vs.20nat_cast/near/346323810 first.

[YaelDillies]

Seems like there's a majority in favour

[eric-wieser]

Please make a PR to update the naming guide to explicitly record this clarification to the naming convention; if we're going to make this sweeping a change, we should make sure it's documented as the preferred naming convention.

[YaelDillies]

https://github.com/leanprover-community/leanprover-community.github.io/pull/459

[kim-em]

bors d+

[mathlib-bors[bot]]

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[YaelDillies]

bors merge

[mathlib-bors[bot]]

Pull request successfully merged into master.

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