libexpr: Add EvalProfiler

This patch adds an EvalProfiler and MultiEvalProfiler that can be used
to insert hooks into the evaluation for the purposes of function tracing
(what function-trace currently does) or for flamegraph/tracy profilers.

See the following commits for how this is supposed to be integrated into
the evaluator and performance considerations.

src/libexpr/eval-profiler.cc

@@ -0,0 +1,48 @@
+#include "nix/expr/eval-profiler.hh"
+#include "nix/expr/nixexpr.hh"
+
+namespace nix {
+
+void EvalProfiler::preFunctionCallHook(
+    const EvalState & state, const Value & v, std::span<Value *> args, const PosIdx pos)
+{
+}
+
+void EvalProfiler::postFunctionCallHook(
+    const EvalState & state, const Value & v, std::span<Value *> args, const PosIdx pos)
+{
+}
+
+void MultiEvalProfiler::preFunctionCallHook(
+    const EvalState & state, const Value & v, std::span<Value *> args, const PosIdx pos)
+{
+    for (auto & profiler : profilers) {
+        if (profiler->getNeededHooks().test(Hook::preFunctionCall))
+            profiler->preFunctionCallHook(state, v, args, pos);
+    }
+}
+
+void MultiEvalProfiler::postFunctionCallHook(
+    const EvalState & state, const Value & v, std::span<Value *> args, const PosIdx pos)
+{
+    for (auto & profiler : profilers) {
+        if (profiler->getNeededHooks().test(Hook::postFunctionCall))
+            profiler->postFunctionCallHook(state, v, args, pos);
+    }
+}
+
+EvalProfiler::Hooks MultiEvalProfiler::getNeededHooksImpl() const
+{
+    Hooks hooks;
+    for (auto & p : profilers)
+        hooks |= p->getNeededHooks();
+    return hooks;
+}
+
+void MultiEvalProfiler::addProfiler(ref<EvalProfiler> profiler)
+{
+    profilers.push_back(profiler);
+    invalidateNeededHooks();
+}
+
+}