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GH-39138: [R] Fix implicit conversion warnings (#39250)

### Rationale for this change

We have failing CRAN checks because this warning occurs on one check machine.

### What changes are included in this PR?

Implicit integer casts are made explicit and/or variable declarations were fixed so that fewer implicit integer casts were performed. Fully solving the warnings also requires https://github.com/r-lib/cpp11/pull/349 since some errors occur in those headers.

### Are these changes tested?

This particular test we can't do on CI because the MacOS runner we have doesn't have a new enough `clang` to support the requisite `-W` flags. I tested this locally by adding `PKG_CXXFLAGS=-Wconversion -Wno-sign-conversion -Wsign-compare -Werror` to `Makevars.in`.

### Are there any user-facing changes?

No
* Closes: #39138

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Dewey Dunnington <dewey@fishandwhistle.net>