Merge memory.[ch] into string.[ch]

The standard C reference contains memset() and memcpy() function
prototypes in <string.h>, so in order to conform closer to existing
standards, the memory.[ch] sources have been merged into string.

Addresses issue #2.

    #2: memory.c and memory.h are unnecessary

    The functions memory_copy() and memory_set() should be moved into
    the nos/util.[ch] source pair, with their current parent memory.[ch]
    files being removed. Having those two memory utility functions in a
    seperate file does not provide any useful or meaning code
    modularisation, and the files are ambiguously named (what kind of
    memory? What can I expect to gain from adding #include
    <nos/memory.h> to my source file?  Heap functions? malloc?).
string: memory_set and memory_copy return pointers
string: Rename memory_set->memset, memory_copy->memcpy