Remove Bytes, FromBytes, Labels, EntityLabels. Document rest of bevy_…

…core and enable warning on missing docs. (#3521)

This PR is part of the issue #3492.

# Objective

 - Clean up dead code in `bevy_core`.
 - Add and update the `bevy_core` documentation to achieve a 100% documentation coverage.
 - Add the #![warn(missing_docs)] lint to keep the documentation coverage for the future.

# Solution

 - Remove unused `Bytes`, `FromBytes`, `Labels`, and `EntityLabels` types and associated systems.
 - Made several types private that really only have use as internal types, mostly pertaining to fixed timestep execution.
 - Add and update the bevy_core documentation.
 - Add the #![warn(missing_docs)] lint.

# Open Questions

Should more of the internal states of `FixedTimestep` be public? Seems mostly to be an implementation detail unless someone really needs that fixed timestep state.

crates/bevy_core/src/float_ord.rs

@@ -1,4 +1,3 @@
-use crate::bytes_of;
 use std::{
     cmp::Ordering,
     hash::{Hash, Hasher},
@@ -42,12 +41,12 @@ impl Hash for FloatOrd {
     fn hash<H: Hasher>(&self, state: &mut H) {
         if self.0.is_nan() {
             // Ensure all NaN representations hash to the same value
-            state.write(bytes_of(&f32::NAN))
+            state.write(bytemuck::bytes_of(&f32::NAN))
         } else if self.0 == 0.0 {
             // Ensure both zeroes hash to the same value
-            state.write(bytes_of(&0.0f32))
+            state.write(bytemuck::bytes_of(&0.0f32))
         } else {
-            state.write(bytes_of(&self.0));
+            state.write(bytemuck::bytes_of(&self.0));
         }
     }
 }