refactor(tvix): always pass Bindings by ptr, use shared/unique_ptr

Value now carries a shared_ptr<Bindings>, and all Bindings constructors return a unique_ptr<Bindings>.

The test that wanted to compare two Bindings by putting them into Values has been modified to use the new Equal() method on Bindings (extracted from EvalState).

Change-Id: I8dfb60e65fdabb717e3b3e5d56d5b3fc82f70883
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1744
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

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

@@ -8,6 +8,6 @@
 namespace nix {
 
 Value* findAlongAttrPath(EvalState& state, const std::string& attrPath,
-                         Bindings& autoArgs, Value& vIn);
+                         Bindings* autoArgs, Value& vIn);
 
 }