libexpr: Add SampleStack stack-sampling profiler

This patch adds support for a native stack sampling
profiler to the evaluator, which saves a collapsed stack
profile information to a configurable location.

Introduced options (in `EvalSettings`):

- `eval-profile-file` - path to the collected profile file.
- `eval-profiler-frequency` - sampling frequency.
- `eval-profiler` - enumeration option for enabling the profiler.

  Currently only `flamegraph` is supported, but having this an
  enumeration rather than a boolean switch leaves the door open
  for other profiler variants (e.g. tracy).

Profile includes the following information on best-effort basis (e.g. some lambdas might
have an undefined name). Callstack information contains:

- Call site location (where the function gets called).
- Primop/lambda name of the function being called.
- Functors/partial applications don't have a name attached to them unlike special-cased primops and lambads.

For cases where callsite location isn't available we have to resort to providing
the location where the lambda itself is defined. This removes some of the confusing
`«none»:0` locations in the profile from previous attempts.

Example usage with piping directly into zstd for compression:

```
nix eval --no-eval-cache nixpkgs#nixosTests.gnome \
  --eval-profiler flamegraph \
  --eval-profile-file >(zstd -of nix.profile.zstd)
```

Co-authored-by: Jörg Thalheim <joerg@thalheim.io>

src/libexpr/eval-profiler-settings.cc

@@ -0,0 +1,49 @@
+#include "nix/expr/eval-profiler-settings.hh"
+#include "nix/util/configuration.hh"
+#include "nix/util/logging.hh" /* Needs to be included before config-impl.hh */
+#include "nix/util/config-impl.hh"
+#include "nix/util/abstract-setting-to-json.hh"
+
+#include <nlohmann/json.hpp>
+
+namespace nix {
+
+template<>
+EvalProfilerMode BaseSetting<EvalProfilerMode>::parse(const std::string & str) const
+{
+    if (str == "disabled")
+        return EvalProfilerMode::disabled;
+    else if (str == "flamegraph")
+        return EvalProfilerMode::flamegraph;
+    else
+        throw UsageError("option '%s' has invalid value '%s'", name, str);
+}
+
+template<>
+struct BaseSetting<EvalProfilerMode>::trait
+{
+    static constexpr bool appendable = false;
+};
+
+template<>
+std::string BaseSetting<EvalProfilerMode>::to_string() const
+{
+    if (value == EvalProfilerMode::disabled)
+        return "disabled";
+    else if (value == EvalProfilerMode::flamegraph)
+        return "flamegraph";
+    else
+        unreachable();
+}
+
+NLOHMANN_JSON_SERIALIZE_ENUM(
+    EvalProfilerMode,
+    {
+        {EvalProfilerMode::disabled, "disabled"},
+        {EvalProfilerMode::flamegraph, "flamegraph"},
+    });
+
+/* Explicit instantiation of templates */
+template class BaseSetting<EvalProfilerMode>;
+
+}