{libexpr,libcmd}: Make debugger significantly faster

The underlying issue is that debugger code path was
calling PosTable::operator[] in each eval method.
This has become incredibly expensive since 5d9fdab3de.

While we are it it, I've reworked the code to
not use std::shared_ptr where it really isn't necessary.

As I've documented in previous commits, this is actually
more a workaround for recursive header dependencies now
and is only necessary in `error.hh` code.

Some ad-hoc benchmarking:

After this commit:

```
Benchmark 1: nix eval nixpkgs#hello --impure --ignore-try --no-eval-cache --debugger
  Time (mean ± σ):     784.2 ms ±   7.1 ms    [User: 561.4 ms, System: 147.7 ms]
  Range (min … max):   773.5 ms … 792.6 ms    10 runs
```

On master 3604c7c51:

```
Benchmark 1: nix eval nixpkgs#hello --impure --ignore-try --no-eval-cache --debugger
  Time (mean ± σ):     22.914 s ±  0.178 s    [User: 18.524 s, System: 4.151 s]
  Range (min … max):   22.738 s … 23.290 s    10 runs
```

src/libexpr/eval-error.cc

@@ -45,7 +45,7 @@ EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrame(const Env & env, const Expr
     // TODO: check compatibility with nested debugger calls.
     // TODO: What side-effects??
     error.state.debugTraces.push_front(DebugTrace{
-        .pos = error.state.positions[expr.getPos()],
+        .pos = expr.getPos(),
         .expr = expr,
         .env = env,
         .hint = HintFmt("Fake frame for debugging purposes"),