Implement store instructions STA, STX, STY

[Reference](http://www.obelisk.demon.co.uk/6502/reference.html#STA):

> STA - Store Accumulator
> Stores the contents of the accumulator into memory.
> 
> STX - Store X Register
> Stores the contents of the X register into memory.
> 
> STY - Store Y Register
> Stores the contents of the Y register into memory.

No flags are affected.