Add support for equivalent of C's ptr->field

[densh]

Finding an equivalent for this in Scala is complicated:

1. We could use a combination of Dynamic and whitebox macros but that would make it unusable in IDEs and not portable to dotty. The syntax would be ptr.field which is not the same but close enough.
2. If we try to use exactly the same syntax as in C ptr->field, it gets parsed as ptr.->(field), meaning that field has to be in scope, defined somewhere else. I don't really see how can one make it happen without some dark arcane magic (untyped macros?).

Workaround: (!ptr).field. This will load a whole struct and then extract a field from it.

[densh]

I've chatted with IntelliJ guys on Scala Days and they say it might be possible to add support for derivation of field types through an official extension mechanism they have. So using a whitebox macro for this seems to be the best way to go so far.

[nafg]

@densh instead of Dynamic, for one extra character how about use Symbols: ptr -> 'field

BTW I don't know C++ so I can only guess what it does

[densh]

The eventual solution to this one landed in #472. We make structs anonymous by default and one can access fields via ptr._N where N is the index of the field.