Make `show_stats` only print out heap funcs that have non-zero calls

[Arinerron]

Statistics:
... mallocs count: 7
... callocs count: 1
... frees count: 3
... reallocs count: 0
... reallocarrays count: 0

|
|
V

Statistics:
... mallocs count: 7
... callocs count: 1
... frees count: 3

[Arinerron]

This should also make it easier to track valloc (#40) if we need to (cause otherwise I'd be tempted not to add it since it'd take up more space and should rarely be used anyway)