Fix to my_mergesort so that it sorts properly and by reference.

The main my_mergesort function now copies over the resulting DArray to the
original array.
Fix to a logical error in incrementing the start of the left and right DArrays.
Attempt to get mergesort and heapsort lib functions.

Try to link the bsd libraray and use their merge and heap sorts. Does not work currently. Will come back later.
Add DArray_sort_add into darray algos.

DArray_sort_add pushes a new element onto the darray then sorts it using
the system based qsort. Included in the algorithm file because that's
where all the sorts are defined.
Add bstrlib and associated tests. Up to bsplit.

Put down perfunctory tests for the bstrlib file according to lcthw 36.