make it easy to configure cargo features from nix

Users of the nix package can now just use `.override` to choose what
features they want.

This also makes RocksDB automatically use jemalloc when Conduit is
configured to use jemalloc.

nix/pkgs/default/default.nix

@@ -1,4 +1,4 @@
-# Keep sorted
+# Dependencies (keep sorted)
 { craneLib
 , inputs
 , lib
@@ -6,13 +6,24 @@
 , rocksdb
 , rust
 , stdenv
+
+# Options (keep sorted)
+, default-features ? true
+, features ? []
 }:
 
 let
-  env = {
+  env =
-    CONDUIT_VERSION_EXTRA = inputs.self.shortRev or inputs.self.dirtyShortRev;
+    let
-    ROCKSDB_INCLUDE_DIR = "${rocksdb}/include";
+      rocksdb' = rocksdb.override {
-    ROCKSDB_LIB_DIR = "${rocksdb}/lib";
+        enableJemalloc = builtins.elem "jemalloc" features;
+      };
+    in
+    {
+      CONDUIT_VERSION_EXTRA =
+        inputs.self.shortRev or inputs.self.dirtyShortRev;
+      ROCKSDB_INCLUDE_DIR = "${rocksdb'}/include";
+      ROCKSDB_LIB_DIR = "${rocksdb'}/lib";
     }
     //
     (import ./cross-compilation-env.nix {
@@ -44,6 +55,14 @@ craneLib.buildPackage rec {
     ];
   };
 
+  cargoExtraArgs = "--locked "
+    + lib.optionalString
+      (!default-features)
+      "--no-default-features "
+    + lib.optionalString
+      (features != [])
+      "--features " + (builtins.concatStringsSep "," features);
+
   # This is redundant with CI
   doCheck = false;