RFC: remove native types

[marijnh]

We have unsafe pointers, and we have zero-variant enums. This seems a good alternative for opaque pointers:

enum foreign_object {}
fn do_foreign_things(o: *foreign_object) { ... }

For non-opaque things, just using *u8 or *record_type is also clearer than using a native type.

Please weigh in if you think they have some merits and should be kept.

[graydon]

I'm ok with this change.

[nikomatsakis]

likewise.

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