Add design for structured accounts as an alternative to Generalized Ownership

[aeyakovenko]

Problem

Current implementations of ownership rely on account pubkeys, which cannot be composed across programs.

How these problems manifest in Ethereum. https://medium.com/@kelvinfichter/looking-at-ownership-in-the-evm-6e6914d341d

This is an alternative to #3216, which defines how programs can interoperate strictly inside the data context of the Accounts. The downside of #3216 approach is that performance improvements like Credit-Only accounts are harder to apply.

Summary of Changes

Propose a design that defines a specific structure for accounts that is generalized between many programs.

Three kinds of accounts are proposed:

• UserAccounts, for users that hold tokens and state
• ProgramAccounts, which define the functions that compose the program
• FunctionAccount, which define the executable bytecode for programs

CC #3266 #3216 @garious

[aeyakovenko]

this won't work. the index stored in ProgramAccounts blows up to contain all the (Account,Program,Token,State)