be50b7e UPM experiment

git-subtree-dir: Svelto.ECS
git-subtree-split: be50b7e8af

f0fd419 ignore unity .meta files

git-subtree-dir: Svelto.ECS
git-subtree-split: f0fd419851

changes (random order of importance):
New Serialization framework
more thorough disposing of the EnginesRoot
an EnginesRoot reference should never be held, unless it’s a weak reference. The code changed to stick to this rule
IReactOnAddAndRemove callbacks are now guaranteed to be called after all the entity structs generated by the same entity have been added and before any is removed. both functions pass the EGID of the analysing entity by parameter now, so that the entity struct won’t need to implement INeedEGID for this sole purpose.
The IReactOnSwap MovedFrom method has been removed, it is now redundant.
Entities built or removed during the IReactOnAddAndRemove callbacks are now added and removed immediately and not on the next submission like used to happen. This avoid some awkward checks that were previously needed inside engines.
EntityStreams can get (optionally) the EGID of the entity published, so that the EntityStruct won’t need an INeedEGID for this sole purpose.
Groups are not trimmed anymore when they are emptied to avoid allocations.
Removed a bunch of run-time allocations that weren’t supposed to happen in Release and/or when the Profile define is used in editor (for debugging reasons Svelto.ECS may need to use strings at run-time, but Svelto.ECS is allocation zero in Release and when the Profile keyword is used)
A improved the DynamicEntityDescriptor and ExtendibleEntityDescriptor code and more notably, introduced the new method ExtendedWith<> to facilitate the writing of modular and reusable entity descriptors.
Several minor code design improvements/optimisations

entity structs cannot support strings anymore (use ECSString instead)

Added the concept of QueryUniqueEntity, but don't rely on it, I am still making up my mind on it. If I keep it, it will be confined to EntityStructs only
Added more ECS common components

clean up code

fix code for entity operation submission
clarify variable names

renamed UnityEntiySubmissionScheduler

force to inherit from sequencer to improve readibility

DispatchOnChange and DispatchOnSet must receive an EGID (sorry)
Rename GenericEntityDescriptorHolder to UnityEntityDescriptorHolder

remove some redundant methods