storage: Iterator codegen

[fdymylja]

The codegen misses the type safe iterator codegen.

We should do it after we tackle: #118

[ethanfrey]

I think primary key type-safe iterator is a good first step and used in many places.
The secondary keys/indexes (#118) can come later.

That is how we built cw-plus as well... it was several months of using primary key iterator before we started implementing secondary indexes or more complex structures.

[ethanfrey]

Gone to archway repo