doc: Un-ignore lots of guide-ffi tests

Using some strategically-placed `#` markers most of the examples are testable
(and their contents are nontrivial).

Closes #13445
auto merge of #13459 : alexcrichton/rust/stop-ignoring-ffi, r=sfackler

Using some strategically-placed `#` markers most of the examples are testable
(and their contents are nontrivial).

Closes #13445