Implement an ICache

[mbitsnbites]

The ICache is more important than the DCache for most applications, and it is easier to implement.

Having an ICache will leave the shared memory bus free for the data interface most of the time, letting the instruction fetch stage run uninterrupted even during data operations.

An ICache is also very useful for systems with slow memory (e.g. SDRAM). More so than a DCache since the CPU needs one instruction per clock cycle, while it may not perform data accesses on every clock cycle.