std::arena doesn't run destructors

[msullivan]

Right now, our arena doesn't run destructors. It shouldn't be too hard to fix this: add a drop instrinsic, and then call them from the arena destructor.

[eholk]

You probably don't even need the drop intrinsic. See the implementation of vec::shift, vec::consume, etc.

[msullivan]

Oh, hm. The arena isn't typed, so the interface needs to change to pass in tydescs.