Allow natives to return structs and arrays.

[dvander]

Natives can now return enum structs and fixed-size arrays. The address is passed as a hidden first parameter. For example:

native Struct DoStuff(int x, int y);

Will have:

params[0] = 3
params[1] = the address of the output struct
params[2] = x
params[3] = y

The native MUST return params[1] on success, otherwise, behavior is undefined.

It is recommended that when defining C++ definitions of SourcePawn structs, to use #pragma pack(push, 1) and #pragma pack(pop). In addition, the only types that should appear are "cell_t" and "float". No other type should be used for scalars. For arrays, "char" may be used, but not as a scalar.

When using char arrays, the C++ definition must be careful to convert the char size to a cell-aligned size. For example:

enum struct MyStruct {
    int x;
    int y;
    char message[50];
}

Should look like this in C++:

#pragma pack(push, 1)
struct MyStruct {
    cell_t x;
    cell_t y;
    char message[CharArraySize<50>::bytes];
};
#pragma pack(pop)