docs: write small guide for using pyosys

docs/source/code_examples/pyosys/pass.py

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+from pyosys import libyosys as ys
+
+class AllEnablePass(ys.Pass):
+	def __init__(self):
+		super().__init__(
+			"all_enable",
+			"makes all _DFF_P_ registers require an enable signal"
+		)
+
+	def execute(self, args, design):
+		ys.log_header(design, "Adding enable signals\n")
+		ys.log_push()
+		top_module = design.top_module()
+
+		if "\\enable" not in top_module.wires_:
+			enable_line = top_module.addWire("\\enable")
+			enable_line.port_input = True
+			top_module.fixup_ports()
+
+		for cell in top_module.cells_.values():
+			if cell.type != "$_DFF_P_":
+				continue
+			cell.type = "$_DFFE_PP_"
+			cell.setPort("\\E", ys.SigSpec(enable_line))
+		ys.log_pop()
+
+p = AllEnablePass() # register the pass
+
+# using the pass
+
+design = ys.Design()
+ys.run_pass("read_verilog tests/simple/fiedler-cooley.v", design)
+ys.run_pass("hierarchy -check -auto-top", design)
+ys.run_pass("synth", design)
+ys.run_pass("all_enable", design)
+ys.run_pass("write_verilog out.v", design)
+ys.run_pass("synth_ice40 -json out.json", design)