Add some basic profiling support to the evaluator
Setting the environment variable NIX_COUNT_CALLS to 1 enables some
basic profiling in the evaluator.  It will count calls to functions
and primops as well as evaluations of attributes.
For example, to see where evaluation of a NixOS configuration spends
its time:
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 ./src/nix-instantiate/nix-instantiate '<nixos>' -A system --readonly-mode
...
calls to 39 primops:
    239532 head
    233962 tail
    191252 hasAttr
...
calls to 1595 functions:
    224157 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:19'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:14'
    221767 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/lists.nix:17:10'
...
evaluations of 7088 attributes:
    167377 undefined position
    132459 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:119:41'
     47322 `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/attrsets.nix:13:21'
...
			
			
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		|  | @ -17,6 +17,9 @@ namespace nix { | |||
| #define foreach(it_type, it, collection)                                \ | ||||
|     for (it_type it = (collection).begin(); it != (collection).end(); ++it) | ||||
| 
 | ||||
| #define foreach_reverse(it_type, it, collection)                                \ | ||||
|     for (it_type it = (collection).rbegin(); it != (collection).rend(); ++it) | ||||
| 
 | ||||
| 
 | ||||
| /* Return an environment variable. */ | ||||
| string getEnv(const string & key, const string & def = ""); | ||||
|  |  | |||
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