Create a AccountBuilder for easy creation of complex and valid Accounts.

[OxidizedLoop]

An AccountBuilder, or at least improving the new method would improve the usability of the crate.

The term valid is mentioned in the Account documentation but making it a bool field and making methods interact with it, as well as make methods that interact with is the first step

• Make a valid field #35

Depending on how Account is improved by the valid field, if deemed necessary an AccountBuilder can be created

• Create an AccountBuilder