Add transform hierarchy stress test (bevyengine#4170)

## Objective

There recently was a discussion on Discord about a possible test case for stress-testing transform hierarchies.

## Solution

Create a test case for stress testing transform propagation.

*Edit:* I have scrapped my previous example and built something more functional and less focused on visuals.

There are three test setups:

- `TestCase::Tree` recursively creates a tree with a specified depth and branch width
- `TestCase::NonUniformTree` is the same as `Tree` but omits nodes in a way that makes the tree "lean" towards one side, like this:
  <details>
  <summary></summary>

  ![image](https://user-images.githubusercontent.com/3957610/158069737-2ddf4e4a-7d5c-4ee5-8566-424a54a06723.png)
  </details>
- `TestCase::Humanoids` creates one or more separate hierarchies based on the structure of common humanoid rigs
  - this can both insert `active` and `inactive` instances of the human rig

It's possible to parameterize which parts of the hierarchy get updated (transform change) and which remain unchanged. This is based on @james7132 suggestion:
There's a probability to decide which entities should remain static. On top of that these changes can be limited to a certain range in the hierarchy (min_depth..max_depth).

Cargo.toml

@@ -592,3 +592,9 @@ icon = "@mipmap/ic_launcher"
 build_targets = ["aarch64-linux-android", "armv7-linux-androideabi"]
 min_sdk_version = 16
 target_sdk_version = 29
+
+# Stress Tests
+[[example]]
+name = "transform_hierarchy"
+path = "examples/stress_tests/transform_hierarchy.rs"
+

examples/README.md

@@ -66,6 +66,7 @@ git checkout v0.4.0
   - [WASM](#wasm)
     - [Setup](#setup-2)
     - [Build & Run](#build--run-2)
+- [Stress Tests](#stress-tests)
 
 # The Bare Minimum
 
@@ -420,3 +421,17 @@ ruby -run -ehttpd examples/wasm
 To load assets, they need to be available in the folder examples/wasm/assets. Cloning this
 repository will set it up as a symlink on Linux and macOS, but you will need to manually move
 the assets on Windows.
+
+# Stress Tests
+
+These examples are used to test the performance and stability of various parts of the engine in an isolated way.
+
+Due to the focus on performance it's recommended to run the stress tests in release mode:
+
+```sh
+cargo run --release --example <example name>
+```
+
+Example | File | Description
+--- | --- | ---
+`transform_hierarchy.rs` | [`stress_tests/transform_hierarchy.rs`](./stress_tests/transform_hierarchy.rs) | Various test cases for hierarchy and transform propagation performance