Rewrite index(), modify insert(), def remove().

data_structures/open_addressed_hash_table/open_addressed_hash_table.py
	-Rewrite `index()` to return either the index of the found item,
	or the first occurrence of vacant/None, such that it's
	generalized for both `insert()` and `remove()`.
	-Rewrite `insert()` accordingly.
	-Implement `remove()`.