Inline payment_plan

This module predates Accounts. That was a separate module because
it used to be part of Bank and those types could be sent to any
smart contract. Now each instruction processor defines for itself
what instructions it accepts.

programs/budget_api/src/budget_processor.rs

@@ -1,7 +1,7 @@
 //! budget program
+use crate::budget_expr::Witness;
 use crate::budget_instruction::BudgetInstruction;
 use crate::budget_state::{BudgetError, BudgetState};
-use crate::payment_plan::Witness;
 use bincode::{deserialize, serialize};
 use chrono::prelude::{DateTime, Utc};
 use log::*;