Implemented scatterUnitWB and gatherUnitWB

[lmbollen]

This PR implements a wishbone addressable scatter unit and a wishbone addressable gather unit.

TODO:

• Implement generic scatter unit
• Implement generic gather unit.
• Implement wishbone interface for the generic scatter and gather units.
• Documentation
• Testing

[christiaanb]

I don't understand why the scatter memory has a 32bit WB bus, but the calendar has a configurable-width WB bus. Why not?:

• Both configurable, or
• Both 32 bit

[lmbollen]

We know that:

• The framewidth for now will be 64 bits since we have to send over the timing oracles(f.k.a. UGNs).
• The wishbone bus interface that accesses the s/g units will most likely be 32 bits since they're hooked up to rv32imc.

By enforcing the s/g unit bus to be 32 bits we can have a nice way to interface with the 64 bit s/g units (either using the upper or lower 32 bits).
However, the calendar wishbone bus has no requirements to constrain the amount of bytes on the bus and may not be hooked up to the same processor that the s/g bus is hooked up to.

To summarize:

• The s/g bus cannot be easily made configurable due to the 64 bit framewidth constraint
• There is no reason other than readability(?) to constrain the calendar bus to 32 bits.

[christiaanb]

Why not reuse?: https://github.com/google-research/bittide/blob/b2a384339681a9bb14e191b5a715c49be9e46af7/bittide/tests/Tests/Calendar.hs#L82-L131

[lmbollen]

The generator in Tests.Calendar requires a supplied generator for the elements, but for the scatter / gather tests, you want the entries in the calendar to be unique, which therefor uses a shuffled version of an incrementing list.

[martijnbastiaan]

I think your test cases would be more readable if you export the the pre-type level fuzzed parts to top-level.