Scratch pad for stuff to do before merging master
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restore test_runner.zig to master branch
 - also the default panic function and unexpected_error_tracing. see the commit
   that adds this text to BRANCH_TODO file.
 - and std/specia/bootstrap.zig

get an empty file compiling successfully (with no panic fn override)

uncomment all the behavior tests

look at all the ir_gen_node ir_gen_node_extra calls and make sure result locations are properly propagated
            return ir_gen_comptime(irb, scope, node, lval);