Too many places memories are lowered in firrtl

as mentioned in #4687, memory lowering is spread through multiple passes in firrtl (seqmems, lowertypes, lowertohw, etc). We need to consolidate this concern to a pass.

Start by reading issue #4687 and tracing memory lowering across the mentioned firrtl passes: seqmems, lowertypes, and lowertohw. Determine the intended single pass boundary and what each existing pass currently contributes. Done means the memory-lowering concern is consolidated into one pass without leaving duplicated lowering responsibilities across those passes.

Written by the indexing model from the issue text.