Add `MemoryData`; like `Signal` but for memories

[whitequark]

Right now the memory geometry (shape/width and depth) are set either on MemoryInstance (the internal perma-unstable primitive) or on lib.memory.Memory (the user-facing interface). However, this creates a problem for the simulator:

• MemoryInstance is not really usable for most code, so whatever it's doing is irrelevant for 99% of users.
• lib.memory.Memory can't be directly read using sim.memory_{read,write} from RFC 36; those primitives only work on MemoryIdentity, which is a marker class whose only purpose is to have an identity.
  • Also we have no primitives for calling sim.memory_{read,write} on lib.memory.Memory at the moment.
• Neither lib.memory.Memory nor MemoryInstance can be passed to traces=[], you have to pass a MemoryInstance.

To solve this, I think we should have a MemoryData(shape: ShapeLike, depth: int, init: MemoryData.Init) class, which is essentially the 2d counterpart of Signal(). Once we add this, we:

• Remove/replace MemoryIdentity with MemoryData.
• Teach sim.memory_{read,write} to use MemoryData.
• Teach write_vcd(traces=[...]) to use MemoryData.
• Add a lib.memory.Memory(data=) constructor that accepts MemoryData.
  • Unclear whether the existing constructor lib.memory.MemoryData(shape=, depth=) should be kept.

[wanda-phi]

RFCd as amaranth-lang/rfcs#62

[whitequark]

Superseded by #1241.