remove template arguments from NimArr constructors and destructors for C++20

[perrydv]

This attempts to fix the recent new CRAN R-devel warnings on fedora-gcc.

@paciorek Do you have a way to test whether this resolves the CRAN testing warnings on that platform?

[paciorek]

Not sure - we could try rhub along the following lines (but choosing the platform option that makes most sense),

Rscript -e "library(rhub);check(platform = c('ubuntu-gcc-devel','windows-x86_64-devel','linux-x86_64-rocker-gcc-san','fedora-clang-devel'))" >> /tmp/rhub.out

but when I last tried it I ran this, so we'd have to dig into that:

As of 2024-06-06, having SSL certificate issues. Also it looks like rhub has updated its workflow substantially so a bit of work may be needed to get this working again.
Error in curl::curl_fetch_memory(url, handle = handle) : 
  SSL peer certificate or SSH remote key was not OK: [builder.r-hub.io] SSL certificate problem: self-signed certificate

I'm happy to look into it later this week (have a retreat tomorrow and Wednesday).

[paciorek]

Per email discussion (and updated info in RELEASE_INSTRUCTIONS) I got rhub working again, and specifically use the gcc14 platform to verify that nimble builds without warnings with the fixes in this PR.