Fix segfault on infinite recursion in some cases

This fixes a segfault on infinite function call recursion (rather than
infinite thunk recursion) by tracking the function call depth in
`EvalState`.

Additionally, to avoid printing extremely long stack traces, stack
frames are now deduplicated, with a `(19997 duplicate traces omitted)`
message. This should only really be triggered in infinite recursion
scenarios.

Before:

    $ nix-instantiate --eval --expr '(x: x x) (x: x x)'
    Segmentation fault: 11

After:

    $ nix-instantiate --eval --expr '(x: x x) (x: x x)'
    error: stack overflow

           at «string»:1:14:
                1| (x: x x) (x: x x)
                 |              ^

    $ nix-instantiate --eval --expr '(x: x x) (x: x x)' --show-trace
    error:
           … from call site
             at «string»:1:1:
                1| (x: x x) (x: x x)
                 | ^

           … while calling anonymous lambda
             at «string»:1:2:
                1| (x: x x) (x: x x)
                 |  ^

           … from call site
             at «string»:1:5:
                1| (x: x x) (x: x x)
                 |     ^

           … while calling anonymous lambda
             at «string»:1:11:
                1| (x: x x) (x: x x)
                 |           ^

           … from call site
             at «string»:1:14:
                1| (x: x x) (x: x x)
                 |              ^

           (19997 duplicate traces omitted)

           error: stack overflow
           at «string»:1:14:
                1| (x: x x) (x: x x)
                 |              ^

src/libexpr/eval.hh

@@ -622,6 +622,11 @@ private:
         const SourcePath & basePath,
         std::shared_ptr<StaticEnv> & staticEnv);
 
+    /**
+     * Current Nix call stack depth, used with `max-call-depth` setting to throw stack overflow hopefully before we run out of system stack.
+     */
+    size_t callDepth = 0;
+
 public:
 
     /**