Fix IEx.Helpers.process_info/1 heap and stack memory calculation

[AlexGx]

Fix :total_heap_size, :heap_size, :stack_size displayed in wrong units, :erlang.process_info/2 returns it
in VM's word size.

iex> pid = spawn(fn -> receive do after :infinity -> nil end end)
iex> process_info(pid)

# Process #PID<0.110.0>

...

## Memory 

Memory               2 KB
Total heap size      233 B
Heap size            233 B
Stack size           9 B

Here 233, 233, and 9 are words

References
https://www.erlang.org/doc/apps/erts/erlang.html#process_info/2
https://www.erlang.org/doc/apps/erts/garbagecollection.html#sizing-the-heap

[josevalim]

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