Made up for the pebble's lack of implementing calloc and realloc.

Quick and dirty implementation of calloc and realloc, to make up for
Pebble's lack of realloc and calloc methods.

Their response to me asking for endpoints:

> Not in the short term. Our memory heap implementation is simple and
> those would not be very optimized. It is probably better to do it
> yourself if you really need that type of feature. We want to avoid
> people relying on them and expecting them to be very optimized.

I don't necessarily agree with them, but it looks like it needs to be
re-implemented.