Replace parking_lot with atomic_refcell

You can still opt-in to use parking_lot if you plan to use the same
egui::Context from multiple threads.

Cargo.lock

@@ -57,6 +57,12 @@ version = "0.2.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "000444226fcff248f2bc4c7625be32c63caccfecc2723a2b9f78a7487a49c407"
 
+[[package]]
+name = "atomic_refcell"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3bc31dce067eab974c815a9deb95f6217806de7b53685d7fc31f8ccf3fb2539f"
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -579,6 +585,7 @@ name = "egui"
 version = "0.5.0"
 dependencies = [
  "ahash",
+ "atomic_refcell",
  "criterion",
  "parking_lot",
  "rusttype",
@@ -618,7 +625,6 @@ version = "0.5.0"
 dependencies = [
  "egui",
  "js-sys",
- "parking_lot",
  "serde",
  "serde_json",
  "wasm-bindgen",
@@ -853,9 +859,6 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "61124eeebbd69b8190558df225adf7e4caafce0d743919e5d6b19652314ec5ec"
 dependencies = [
  "cfg-if 1.0.0",
- "js-sys",
- "wasm-bindgen",
- "web-sys",
 ]
 
 [[package]]