Synthesis Papercut pass only-read/write checks for `ref` memories

[jiahanxie353]

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The Synthesis Papercut pass incorrectly flags reference memories that are only read or written to as needing @external flags. This happens because the pass does not recognize that reference memories must have external memories associated with them in some other components. Since reading/writing to ref memories has the same effect as reading/writing to @external memories, we don't need to

Add @external to cell to turn this into an interface memory.

As an illustrating example, the following example should be valid:

component main() -> () {
  cells {
    @external(1) x = comb_mem_d1(32, 4, 3);
    st = store();
  }
  wires {}
  control {
    invoke store[ext_mem0 = x]()();
  }
}

component store(@clk clk: 1, @reset reset: 1, @go go: 1) -> (@done done: 1) {
  cells {
    ref ext_mem0 = comb_mem_d1(32, 4, 3)
    std_slice_0 = std_slice(32, 3);
    load_0_reg = std_reg(32);
    ret_arg0_reg = std_reg(32);
  }
  wires {
    out0 = ret_arg0_reg.out;
    group bb0_0 {
      ...
      ext_mem0.addr0 = std_slice_0.out;
      ext_mem0.write_data = load_0_reg.out;
      ...
    }
    group ret_assign_0 { ... }
  }
  control {
    seq {
       bb0_0;
       ret_assign_0;
    }
  }
}