Fix missing value propagation in `as_integers/doubles()`

[DavisVaughan]

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This PR set out with the small goal of fixing as_integers() and as_doubles() to ensure that they propagate missing values from their inputs correctly when performing coercion.

It got a little more complicated along the way, as some forward declarations were needed and some functions had to be shifted around to get things to compile right. I think this is all for the better though, as it revealed a few subtle bugs which I will discuss inline below.

[DavisVaughan]

The format_check build seems to be failing because I switched the include order of cpp11/integers.hpp and cpp11/doubles.hpp in test-integers.cpp. It seems to want to retain them in alphabetical order?

I disagree somewhat strongly with this, because including integers.hpp first in the integers test file should reduce the chances of having hidden dependency bugs, like the one introduced by https://github.com/r-lib/cpp11/pull/265#discussion_r819072026

I can change it back if we really want to get the formatter build to pass, but it seems like an anti pattern.

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The 3.4 build is failing for some unrelated pandoc reasons

[sbearrows]

LGTM!

[DavisVaughan]

I have learned that because we set: https://github.com/r-lib/cpp11/blob/322e5ee67346926993cc6f464152802118250b78/.clang-format#L4

we can separate the includes into "blocks" and they will be sorted within their block. So I added a space between cpp11/integers.hpp and the other includes to block it off.

It we wanted to be more extreme, we could set SortIncludes: Never in the configuration file, but this at least gets us passing again (see https://clang.llvm.org/docs/ClangFormatStyleOptions.html)

[vspinu]

Would be nice to add conversion from logicals as well. Primarly for the sake of all NA vectors.

[romainfrancois]

I'll merge this now and follow up.