make executable, vote and stake account rent exempt (#6017)

* add missing convenience method

* require vote account to be exempt

* make stake account rent exempt

* making executable rent exempt

* rent will be initialized in genesis

* add test for update_rent

sdk/src/sysvar/rent.rs

@@ -49,6 +49,30 @@ pub fn create_account(lamports: u64, rent_calculator: &RentCalculator) -> Accoun
     .unwrap()
 }
 
+use crate::account::KeyedAccount;
+use crate::instruction::InstructionError;
+
+pub fn from_keyed_account(account: &KeyedAccount) -> Result<Rent, InstructionError> {
+    if !check_id(account.unsigned_key()) {
+        return Err(InstructionError::InvalidArgument);
+    }
+    Rent::from_account(account.account).ok_or(InstructionError::InvalidArgument)
+}
+
+pub fn verify_rent_exemption(
+    account: &KeyedAccount,
+    rent_sysvar_account: &KeyedAccount,
+) -> Result<(), InstructionError> {
+    if !from_keyed_account(rent_sysvar_account)?
+        .rent_calculator
+        .is_exempt(account.account.lamports, account.account.data.len())
+    {
+        Err(InstructionError::InsufficientFunds)
+    } else {
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;