#17 Use UHEAPNODE instead of size_t in heap.c

[Moritz Strübe]

Used memory has a header of type size_t. Using a special type UHEAPNODE enables extensions like assigning Heap-Mem to threads without too much hassle.

Sugested type

typedef struct _UHEAPNODE{
size_t hn_size;
} UHEAPNODE;

A patch can be provided on request.