This makes it easier to define schemas when the type name is non-trivial (e.g., templated structs).
This is breaking for a) custom struct implementations and b) anything calling `wpi::Struct<T>::GetTypeString(info...)` in C++ directly. In both cases, it's a simple translation: For A, rename `GetTypeString()` to `GetTypeName()` and remove the struct: at the beginning, and for B, use `wpi::GetStructTypeString<T>(info...)` instead.
This required changing the constant values (e.g. kSize) into functions
(e.g. GetSize()).
Fixed implementations of ForEachNested to be inline (as these are actually
templates).
Also added a ntcore Struct test.