Provide an efficient implementation of ‘elem’

The one in Nixpkgs is O(n^2), this one is O(n).  Big reduction in the
number of list allocations.

Statistics before (on a NixOS system config):

  time elapsed: 1.17982
  size of a value: 24
  environments allocated: 1543334 (39624560 bytes)
  list elements: 9612638 (76901104 bytes)
  list concatenations: 37434
  values allocated: 1854933 (44518392 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18272
  number of thunks: 1392467
  number of thunks avoided: 1507311
  number of attr lookups: 430801
  number of primop calls: 691600
  number of function calls: 1492502

Statistics after:

  time elapsed: 1.08683
  size of a value: 24
  environments allocated: 1384376 (35809568 bytes)
  list elements: 6946783 (55574264 bytes)
  list concatenations: 37434
  values allocated: 1760440 (42250560 bytes)
  attribute sets allocated: 392040
  right-biased unions: 186334
  values copied in right-biased unions: 591137
  symbols in symbol table: 18273
  number of thunks: 1297673
  number of thunks avoided: 1380759
  number of attr lookups: 430802
  number of primop calls: 628912
  number of function calls: 1333544

src/libexpr/primops.cc

@@ -880,7 +880,8 @@ static void prim_head(EvalState & state, Value * * args, Value & v)
 
 
 /* Return a list consisting of everything but the the first element of
-   a list. */
+   a list.  Warning: this function takes O(n) time, so you probably
+   don't want to use it!  */
 static void prim_tail(EvalState & state, Value * * args, Value & v)
 {
     state.forceList(*args[0]);
@@ -930,6 +931,19 @@ static void prim_filter(EvalState & state, Value * * args, Value & v)
 }
 
 
+static void prim_elem(EvalState & state, Value * * args, Value & v)
+{
+    bool res = false;
+    state.forceList(*args[1]);
+    for (unsigned int n = 0; n < args[1]->list.length; ++n)
+        if (state.eqValues(*args[0], *args[1]->list.elems[n])) {
+            res = true;
+            break;
+        }
+    mkBool(v, res);
+}
+
+
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, Value * * args, Value & v)
 {
@@ -1145,6 +1159,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__tail", 1, prim_tail);
     addPrimOp("map", 2, prim_map);
     addPrimOp("__filter", 2, prim_filter);
+    addPrimOp("__elem", 2, prim_elem);
     addPrimOp("__length", 1, prim_length);
 
     // Integer arithmetic