Protip: don't reference a variable you just free()'d :)
Explicitly keep track of total_elements for merging instead of summing first_a_in_b with a->nel etc etc, which saves a variable but is otherwise all downside
memset 0 so you can more easily see the merge process
Verbosity for copying remainders
Verbosity consistency and printing the result of the remainder copying
Fixed offset problems for merging backwards
Added TODO for SUPPRESS_STDOUT macros
Added TODO to find out if both left and right remainders are possible given the bounded merging for the arrays