Add helper methods to EvmContext

[DaniPopes]

Helper wrapper methods can be added to all of the methods in JournaledState that require a DB on EvmContext by just passing along all arguments to self.journaled_state.<method>(..., &mut self.db).

E.g. load_code:

// JournaledState
impl JournaledState {
    pub fn load_code<DB: Database>(
        &mut self,
        address: Address,
        db: &mut DB,
    ) -> Result<(&mut Account, bool), EVMError<DB::Error>> { /* ... */ }
}

// Wrapper in EvmContext
impl<DB: Database> EvmContext<DB> {
    pub fn load_code(
        &mut self,
        address: Address
    ) -> Result<(&mut Account, bool), EVMError<DB::Error>> { // same result
        self.journaled_state.load_code(address, &mut self.db)
    }
}