Most efficient hash functions use uint32/uint64 and produce values
that do not fit in out small nat representation. Thus, GMP big numbers
would have to be created.
The array dimension is now stored inside the array.
The main motivation is that it reflects the actual runtime implementation.
We need to store the array size to be able to GC it.
So, it feels silly to have the array size stored in each array object,
but we cannot use this information.
For example, in the `hashmap` we implemented the bucket array using
`array`, and we store the `size` of the array.
Same for the Lean3 `buffer` object.
The `buffer` object doesn't even need to exist.
The actual `array` implementation is the `buffer`
We use the same approach used to define rbtree:
1- Structure with minimal number of invariants, AND
2- well_formed inductive predicate
We can use the well_formed predicate to prove auxiliary invariants later.
Example: the keys stored in every bucket have the correct hash code.
This implementation does not depend on the tactic framework,
and it is not a mess like the one in mathlib.
