feat(tvix/eval): Implement builtins.mapAttrs

I played around a little bit with doing this in-place, but ended up
going with this perhaps slightly clone-heavy approach for now because
ideally most clones on Value are cheap - but later we should benchmark
alternate approaches that get to reuse allocations better if necessary
or possible.

Change-Id: If998eb2056cedefdf2fb480b0568ac8329ccfc44
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7068
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-mapattrs.exp

@@ -1 +0,0 @@
-{ x = "x-foo"; y = "y-bar"; }

tvix/eval/src/tests/nix_tests/notyetpassing/eval-okay-mapattrs.nix

@@ -1,3 +0,0 @@
-with import ./lib.nix;
-
-builtins.mapAttrs (name: value: name + "-" + value) { x = "foo"; y = "bar"; }