Fail to compile on 16-bit platforms (bevyengine#4736)

# Objective
`bevy_ecs` assumes that `u32 as usize` is a lossless operation and in a few cases relies on this for soundness and correctness. The only platforms that Rust compiles to where this invariant is broken are 16-bit systems.

A very clear example of this behavior is in the SparseSetIndex impl for Entity, where it converts a u32 into a usize to act as an index. If usize is 16-bit, the conversion will overflow and provide the caller with the wrong index. This can easily result in previously unforseen aliased mutable borrows (i.e. Query::get_many_mut).

## Solution
Explicitly fail compilation on 16-bit platforms instead of introducing UB. 

Properly supporting 16-bit systems will likely need a workable use case first.

---

## Changelog
Removed: Ability to compile `bevy_ecs` on 16-bit platforms.

## Migration Guide
`bevy_ecs` will now explicitly fail to compile on 16-bit platforms.  If this is required, there is currently no alternative. Please file an issue (https://github.com/bevyengine/bevy/issues) to help detail your use case.

crates/bevy_ecs/src/lib.rs

@@ -1,5 +1,8 @@
 #![doc = include_str!("../README.md")]
 
+#[cfg(target_pointer_width = "16")]
+compile_error!("bevy_ecs cannot safely compile for a 16-bit platform.");
+
 pub mod archetype;
 pub mod bundle;
 pub mod change_detection;