Tests that classes with dtors and multiple fields work correctly.
Closes #2708

Also fixed shapes for classes with dtors, as well as handling
offsets for classes with dtors correctly in take glue.

Closes #2485

since dtors always have external linkage

issue-912 is xfailed. issue-2111 is already fixed, but it's good to
have the test case.

This was a call to span_bug() before. I'm not sure about the other cases,
but the test case shows that the `vk_self` case can certainly arise with
a bad program, so it should be a span_err() thing and not a span_bug() thing.

Closes #2590

A cleaner way to handle suffix inference for unary minus exprs.

This reverts commit 290206b1.

Forgot to add the actual native code, temporarily reverting.

The first benchmark shows about twice the throughput of the old system.