Reflect trait should not require Send + Sync bounds

[alice-i-cecile]

Bevy version

0.15

Problem

Reflect has no intrinsic need to be Send + Sync. This limits its uses, especially in a broader non-Bevy context.

Solution

Remove Send + Sync trait bounds, adding them locally to any methods that fail to compile.

Context

According to @MrGVSV, this was originally for compatibility with components, before the explicit Component trait was added. This is backed up by Cart here

[BenjaminBrienen]

I'm picking this up now