Add test for writing-through-uninit bug (reported on IRC by jrmuizel), plus fix in typestate system.

Include all lval-writing statements in stmt_is_init calculation, not just "copy-like". Un-XFAIL generic-tag-alt.rs

A certain incomplete quantity of wrestling with "INIT" statements that don't actually initialize. Should probably rename them to MAKE. Anyway, WIP, but two steps forward (and one back). More later.

This involved adding an Ast.ty return to trans_lval.

I also included the code for record and box access, but their tests don't
completely pass yet.

them an argument to new_block.  gdb should do a good job of single-stepping
LLVM-compiled rust programs by rust line number now.

which are helpful in debugging the llvm backend.  Also tell git to ignore
LLVM's intermediate files.

Tidy up handling of unimplemented features. These are expected (if undesirable) sorts of error, we should handle better than "backtrace and exit 2".