Deprecate std_mem

[rachitnigam]

Standard memories are, in general, not synthesizable to BRAMs or URAMs on FPGAs because of combinational reads. The new sequential memories provide sequential, latency-insensitive interfaces to memories and are amenable to mapping optimizations like #1151.

In addition to #907, we should move the std_mem primitives into the unsynthesizable.futil file to signal that they cannot be easily synthesized and used

TODO

• Change CIRCT to use seq memories

[andrewb1999]

Before we do this we should add seq_mems to CIRCT and move passes over to use them instead.

[rachitnigam]

@cgyurgyik thoughts on how hard this would be? The difference is that reads from memories require setting read_en signal and waiting till read_done is high before reading the value

[cgyurgyik]

I imagine the brunt of it will come from refactoring. The PipelineToCalyx pass may be trickier, depending on how specialized memory accesses are. I don't recall exactly.

[rachitnigam]

We can, for now, leave the std_mem in the repo when we do this and write a short pass to emit a warning whenever they are used by a frontend. We can eventually remove them completely once CIRCT stops using them

[rachitnigam]

Linking a conversation that motivated this: #1221

[cgyurgyik]

I think effort towards this was made on the CIRCT side in llvm/circt#5471.

[rachitnigam]

#1901 renamed std_mem_* memories to comb_mem_* to indicate that they have combinational reads. I'm going to consider this sufficient to close this PR. In the future, we can consider removing these memories but for now, we should systematically eliminate their use from the code base.