Various speed improvements

[cpsievert]

Various speed improvements inspired by investigation into rstudio/bslib#369

1. Avoid validation of sass_layer() input when joining two layers together
2. Avoid writing attachments on a sass() cache hit if output hasn't changed
3. Use a faster way to get package installation/versions via {staticimports}
4. Avoid other miscellaneous duplicate work in sass()

[cpsievert]

@wch just FYI, the CI failures are also happening on the main branch -- I'll address those in a separate PR

[wch]

I think the functions need to be re-imported -- the versions in wch/staticimports#11 aren't exactly the same as what's in this PR.

[cpsievert]

Failure to install on Windows 3.6 seem to be an Rtools issue that should be addressed in {shiny-workflows}, so merging