Evaluate Collapsing PropertyCache and VectorCache

[jamescourtney]

The main difference is preallocation of vectors with vector cache. Can we meet in the middle and use a Dictionary that fills up gradually?

[jamescourtney]

Dictionary is too slow to be a proper replacement for an array. However, due to the work in #224, it's now possible to collapse these serialization modes.

PropertyCache, VectorCache, and VectorCacheMutable will be removed in FlatSharp 6. They will be replaced with Progressive and ProgressiveMutable. ProgressiveMutable will support write through for structs.

The semantics of Progressive deserialization are a hybrid of VectorCache and PropertyCache.

• All items are read at most once
• Vectors may be preallocated, but need not be.
• Individual vectors may opt in or out to preallocation via a new attribute (fs_preallocate)

fs_preallocate will support values of Always, Never, and Default, with the following semantics:

	Always	Never	Default
Greedy	OK	Error	OK
Progressive	OK	OK	OK
Lazy	Error	OK	OK

The meaning of Default can vary contextually.

• In a Greedy serializer, Default is equivalent to Always.
• In a Lazy serializer, Default is equivalent to Never.
• In a Progressive serializer, FlatSharp will choose Greedy if the vector is small (ie, < 1024 items or so), and Lazy if the vector is large.

Example:

table Table (fs_serializer:"Progressive")
{
       Vector : [ OtherTable ] (fs_vector:"IIndexedVector", fs_preallocate:"Never");
}

[jamescourtney]

This is done -- need more benchmarking to determine if we should keep the fs_preallocate attribute or not.

[jamescourtney]

fs_preallocate removed.