refactor(tvix): Use absl::btree_map for DerivationOutputs

This container implementation is much faster than std::map. We have
stuck to an ordered container because it's unclear whether the
accesses of this field (of which there are *many*) are actually
ordering dependent.

Also includes an Arbitrary implementation for absl::btree_map (for any
K, V that are also Arbitrary).

Change-Id: I04f58ca0ce32b9ae1759313b01508b0e44bae793
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1683
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/tests/attr-set.cc

@@ -5,6 +5,7 @@
 #include <string>
 #include <vector>
 
+#include <absl/container/btree_map.h>
 #include <bits/stdint-intn.h>
 #include <gc/gc_cpp.h>
 #include <gtest/gtest.h>