Null slots on missing init, null-check box ptrs on drop. Works around missing typestate pass. Un-XFAIL box-in-tup.rs.

rustc: Make copy_args_to_allocas not insist on a function type so it's easier to make it work with tag constructors

rustc: Make new_fn_ctxt and copy_args_to_allocas receive a list of args rather than an AST fn, so we can use it for tag constructors

rustc: Handle n-ary tag constructors in iter_structural_ty (which lets us process their refcounts correctly)

rustc: Change fn_names and fn_ids to item_names and item_ids in trans, since we're soon to be storing tags in there too

rustc: Remove the premature optimization for nullary tag constructors, since it'll be no longer necessary once LLVM gets a unit type