Combinational memories in `gen_exp` and `gen_ntt_pipeline`

[anshumanmohan]

In exactly two places, we use the eDSL to generate combinational and not sequential memories. These are gen_exp and gen_ntt_pipeline. We have discussed that we should use default to sequential memories, and use combinational memories only when we know that the memory is very small. I just wanted to flag these two cases in case we want to revisit them. Porting them over will be very easy.

Originally posted by @anshumanmohan in #2052 (comment)

[rachitnigam]

We should use seq_mems instead.