Make CallBackBank reentrant

[ytmytm]

CallBackBank procedure stores current RAM config from PIA_PORTB in a fixed location. Because of this:

• bank0 procedures can't be used for functions called from interrupts (an interrupt might happen while bank0 procedure (e.g. Rectangle) is running)
• bank0 procedure can call another bank0 procedure only directly in bank0, not via jump table that might route it through CallBankBank

This limits the kind of code that can be moved to bank0 - there is still about 1-2K available there, assuming 4K reserved for SIO driver.