Optimize MaterializationContext

[SergeiPavlov]

Reduce number of fields

Also:

• Convert ItemMaterializationContext into readonly struct
• Remove redundant null-check before TypeIdRegistry[typeInfo]. This indexed accessor already checks for typeInfo==null

[snaumenko-st]

Converting classes to structs like this should be well justified, IMHO. We can't predict how it will influence performance / memory consumption, etc. without benchmarks.