[PoC] Use allowlist to keep the exports minimal

[yutannihilation]

See https://github.com/extendr/libR-sys/issues/95#issuecomment-1037084923

Currently, libR-sys exports all symbols wildly, including those from the C standard library. I think we should keep the exports minimal to avoid confusion. This pull request tries to create a proper allowlist.

Note that this pull request is based on the unmerged branch of https://github.com/extendr/libR-sys/pull/93, so there are a lot of changes irrelevant to this change. The important diff is only this commit: https://github.com/extendr/libR-sys/commit/5f899406fd9a697d553299c1db8d2d1f7209515f