refactor(tvix): completely remove boehm gc

We have decided that leaking memory is a better fate than random,
non-debuggable memory corruption. Future CLs will begin changing
various fields to std::unique_ptr and std::shared_ptr.

It turns out that disabling the GC does not have disasterous impact.
The Nix evaluator only runs on the client CLI, never in any long-
running process. Even the REPL does not leak too badly under this
change, because it uses one EvalState for the duration of the REPL.

Building an explicitly tracing garbage collector is likely in the
future of this project, but that giant amount of work cannot be
done under a nix evaluator that is constantly crashing. We need to
restore development velocity here, and this is the best way we've
figured out to do it.

Change-Id: I2fcda8fcee853c15a9a5e22eca7c5a784bc2bf76
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1720
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

third_party/nix/src/libexpr/attr-set.hh

@@ -2,7 +2,6 @@
 #pragma once
 
 #include <absl/container/btree_map.h>
-#include <gc/gc_allocator.h>
 
 #include "libexpr/nixexpr.hh"
 #include "libexpr/symbol-table.hh"
@@ -14,7 +13,7 @@ class EvalState;
 struct Value;
 
 /* Map one attribute name to its value. */
-struct Attr : public gc {
+struct Attr {
   Symbol name;
   Value* value;  // TODO(tazjin): Who owns this?
   Pos* pos;      // TODO(tazjin): Who owns this?
@@ -22,13 +21,9 @@ struct Attr : public gc {
       : name(name), value(value), pos(pos){};
 };
 
-// Convenience alias for the backing map, with the garbage-collecting
-// allocator explicitly specified.
-using AttributeMap =
-    absl::btree_map<Symbol, Attr, std::less<Symbol>,
-                    traceable_allocator<std::pair<const Symbol, Attr>>>;
+using AttributeMap = absl::btree_map<Symbol, Attr>;
 
-class Bindings : public gc {
+class Bindings {
  public:
   typedef AttributeMap::iterator iterator;