Move maxStackSize to static/class property
Add itemFromString/itemToString

In the tests, strings are used as the item types. But you may want
to use an arbitrary object, in which case having itemFromString is useful
to convert from a string to the custom object type, for fromString() etc.
Remove unused Item class

Out of scope of this module
Rename ItemStack -> ItemPile

Didn't like the computer science connotations of "stack" --
this is not a LIFO data structure, just a grouping of items
(all same type) you can merge and split. Calling it a "pile"
for now.
Move ItemPile to its own module

https://github.com/deathcap/itempile