`document` doesn't react to all files changes

[CGMossa]

Via #246 rextendr::document doesn't re-compile in case you change .cargo/config.toml-file.

The detection of needing to compile comes from pkgbuild. This extra rust feature was provided by way of https://github.com/r-lib/pkgbuild/commit/2fd13d9a3b45332db9a95ab2cf092a5cd1d0dc60

Here it is clearly stated that rs-files and Cargo.toml are the only sensitive ones to recompilation.

There was talk of adding a configurable feature via DESCRIPTION-file, but this was abandoned.

Do we suggest the following additions:

• Recompile if .cargo\config.toml has changed, or .cargo\config? Actually it needs to be **/.cargo/config.toml atleast.
• rust-toolchain.toml file should also provoke a change.
• ???

I'd rather we gather the exact glob-list before making an issue over at pkgbuild.

[Ilia-Kosenkov]

How about we track *rs and *toml? Feels like all of them should affect compilation, no? And by we I mean ask {pkgbuild} :)