Refactor for parties_generators

[ghost-not-in-the-shell]

1. removes all reference to ledger, instead make account_state_tbl a required parameter.
2. provide a function for generation of sequence of parties. This is mainly because generation of sequence of parties needs to decide the set of fee_payer accounts in advance and thread the account_state_tbl through the multiple calls of gen_parties_from.
3. writes a mli files for the interface of parties generators and writes document for those generators.