Add htable_grow()

Add htable_grow()
When initalizing a hash table set a factor that will determine
the limit, beyond of which, the table will grow.

For example, a table with size = 100 and factor = 2,
will grow if it's used items become more than 200.

The strategy we follow for growing is the following:
1) We allocate a new hash table twice as big as the old one
2) We copy all items from the old one to the new one,
rehashing as appropriate
3) We free the old table