Adding autoname and opt_clean -purge Yosys pass to synth.tcl

[mithro]

Fixes #182.

The opt_clean pass removes unused items, the -purge is needed to remove the duplicate private names.

The autoname pass converts the internal _<random number>_ names into something which is a lot more readable (as it is based on the input and outputs).

Before;

  (* src = "adders/top_ff.sv:22.3-23.13" *)
  sky130_fd_sc_hd__dfxtp_1 _47_ (
    .CLK(clk),
    .D(a[8]),
    .Q(a_[8])
  );
  (* src = "adders/top_ff.sv:28.3-29.13" *)
  sky130_fd_sc_hd__dfxtp_1 _48_ (
    .CLK(clk),
    .D(i_[0]),
    .Q(x[0])
  );

After;

  (* src = "adders/top_ff.sv:22.3-23.13" *)
  sky130_fd_sc_hd__dfxtp_1 \a__$_DFF_P__Q  (
    .CLK(clk),
    .D(a[8]),
    .Q(a_[8])
  );
  (* src = "top_ff.sv:28.3-29.13" *)
  sky130_fd_sc_hd__dfxtp_1 \x_$_DFF_P__Q_5  (
    .CLK(clk),
    .D(i_[0]),
    .Q(x[0])
  );