Importing the ~deprecated golang.org/x/exp/maps as "xmaps" to not
shadow the std "maps" was getting ugly.
And using slices.Collect on an iterator is verbose & allocates more.
So copy (x)maps.Keys+Values into our slicesx package instead.
Updates #cleanup
Updates #12912
Updates #14514 (pulled out of that change)
Change-Id: I5e68d12729934de93cf4a9cd87c367645f86123a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
1e2e319e7d