[Seq] Unify `seq`-dialect memory representations

[mortbopet]

As noted in https://github.com/llvm/circt/pull/5009, https://github.com/llvm/circt/pull/5025, seq.firmem was introduced to initially help factoring out memory handling in the FIRRTL-to-HW pass. Once done, it is then intended that it and seq.hlmem can be unified into a single representation - seq.mem - seeing as they both essentially try to solve the same problem.

i.e.

• seq.firmem + seq.hlmem => seq.mem
• seq.firmem.read_port/write_port/read_write_port + seq.read/write/read_write => seq.read_port/write_port/read_write_port