Wrap C++ structs as C# structs where appropriate

[ddobrev]

Structs and classes are essentially the same in C++. However, the way they are used is almost identical as they way they are in C#. So it would be a good idea if a C++ struct is mapped to a C# struct. That is not always convenient - for example, if the struct is inherited, we won't be able to directly mirror that in C# since deriving from structs is forbidden there. Therefore in such cases mapping to a struct would cause more problems than it solves.
So the algorithm should:

1. Only consider C++ structs, that is, types declared with the "struct" keyword;
2. Apply a series of checks to the struct, such as searching the AST for derived types as stated above, to determine if it's beneficial to represent it as a C# struct.

Note: manual marking of types as structs must be left to give freedom to binders.

[genuinelucifer]

@ddobrev Please review #501

[genuinelucifer]

@ddobrev Please review #518

[genuinelucifer]

@ddobrev
There is a corner case for this which can't be checked directly. As in NamespaceDerived and NamespaceBase Test in which both are compiled separately but the Derived one inherits from the Base.
We have practically no way of knowing that a future type (rather compiled separately) will inherit this or not. So, if we marshal NamespaceBase to struct then build will fail for NamespaceDerived.
I could think that if the user enables MarshalStructToStructWhereverPossible() Pass then he must also mark the C++ struct(s) with some macro to prohibit value type marshalling. (ex: STRICT_REF_TYPE)
What do you suggest as a workaround for this?

[genuinelucifer]

@ddobrev Any comment?!

[tomspilman]

Getting this working would be a huge benefit to many APIs cases we've run into. Currently we end up writing a dummy class to wrap the struct with getters/setters to access the fields. It is a lot of wasted effort when a C++ struct can be easily be represented in C#.