Convert PtrToArg macros to regular C++ structs.

[Ivorforce]

I'd like to reduce the number of both macro and template magic throughout Godot. Some of it is pretty convoluted and can be simplified.

As a first step, macros should be converted to regular C++ structs as much as possible.
C style macros are difficult to edit, because they are (until called) not 'regular' code. For example, IDEs struggle to syntax highlight them, or give insights. Therefore, they are undesirable.

For this PR, I simply copy-pasted macro code to named structs. Then, I replaced the macro contents to simply instantiate the requested type with a specialization. Finally, I used my IDE to replace the macros with their contents, to avoid any mistakes with type arguments if I was manually converting.

As a second step, some of these can probably be simplified with C++ template magic in the future. But that's out of scope for this PR.

[Repiteo]

Rather than globally-defining structs with a leading underscore, could we contain them in an Internal namespace instead?

[Ivorforce]

Rather than globally-defining structs with a leading underscore, could we contain them in an Internal namespace instead?

Good idea, done!