Def __init__(), insert(), contains().

data_structures/open_addressed_hash_table/open_addressed_hash_table.py
	-Define the `OpenAddressedHashTable` constructor, as well as
	`insert()` and `contains()`.
	-Additionally define magic methods `__len__()` and `__str__()`,
	to aid in debugging.
Add class/function docstrings, rename contains().

data_structures/open_addressed_hash_table/open_addressed_hash_table.py
	-Add docstrings to `OpenAddressedHashTable` and all of its
	implemented methods.
	-Rename `contains()` to `index()`, to better reflect its
	purpose.