TypeFlattener for nested types

[buybackoff]

TypeFlattener already works quite well for flat structures. Buttwo enhancements are needed:

1. Check if a field is a primitive and keep its value unchanged if it is. Otherwise, check if the type implements IConvertible and convert it according to the GetTypeCode() returned value. If not IConvertible, call ToString(). Spreads' primitive types must implement IConvertible or override ToString() (ToString() override is always needed to avoid boxing).

2. Recursive flattening. Need to decide if IConvertible takes priority over expanding nested non-primitive types. (some logic with attribute presence or custom attributes could apply).