Add support for --with-packing

[DaZombieKiller]

Allows a user to specify an exact value for StructLayoutAttribute.Pack, which is useful for controlling packing with a constant value defined in C#.

For example, --with-packing ExampleStruct=InteropDetails.PackConstant given the following header:

struct ExampleStruct
{
};

Will output the following C# definition of ExampleStruct:

[StructLayout(LayoutKind.Sequential, Pack = InteropDetails.PackConstant)]
public partial struct ExampleStruct
{
}