alloc-fmt: Skip early frames of backtrace

[joshlf]

In alloc-fmt's print_backtrace_and_abort, we currently print the entire backtrace including print_backtrace_and_abort itself and even the anonymous local closure inside that function. It would be good to figure out a way to only begin printing frames once we've reached the parent of print_backtrace_and_abort (which is the function invoking the macro).