Allow user to not store heap data structures in heap itself
    -- rename mdesc struct and export to user: pass in allocated struct to
       umalloc_attach
    -- Find out which members of mdesc are allocated on heap, and provide an
       alternate method (pass in struct with func ptr for allocating new memory
       for data structures: default to using heap itself).

Allow heap to grow down as well as up.
    -- Either as runtime attach() option, or compile two versions of functions.