Give entity_ref its own folder and split into multiple files

[JaySpruce]

Objective

world/entity_ref.rs is the biggest single source file in Bevy at ~6800 lines. It can be a bit hard to navigate.

Solution

Rename the entity_ref module to entity_access and split it into 8 files:

• mod.rs: Re-exports and testing, ~1600 lines
• entity_ref.rs: EntityRef type, ~340 lines
• entity_mut.rs: EntityMut type, ~760 lines
• world_mut.rs: EntityWorldMut type, ~2200 lines
• filtered.rs: FilteredEntityRef and FilteredEntityMut types, ~780 lines
• except.rs: EntityRefExcept and EntityMutExcept types, ~500 lines
• entry.rs: ComponentEntry type, ~330 lines
• component_fetch.rs: DynamicComponentFetch trait, ~360 lines

world_mut is still sizable, but that's just the sheer number of methods EntityWorldMut has. Splitting the methods across multiple files is possible, but that might be more annoying than a large file.