Add Microcode Execution hooks to 6502 Emulator

[ncatelli]

To better support peripheral devices, an event handler should be defined that can be registered with a given cpu prior to the execution of a piece of microcode, i.e. at https://github.com/ncatelli/mainspring/blob/main/src/cpu/mos6502/mod.rs#L204. This should allow direct handling of modifications to a peripheral based on a push from an event rather than requiring the peripheral to constantly poll memory on each tick.