Add example of hamming generator and checker instances

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examples/basic/hamming.py

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+from migen.fhdl import verilog
+from migen.fhdl.std import *
+from migen.genlib.mhamgen import HammingGenerator, HammingChecker
+
+
+# Instantiates Hamming code generator and checker modules back
+# to back.  Also creates an intermediate bus between generator
+# and checker and injects a single-bit error on the bus, to
+# demonstrate the correction.
+class gen_check(Module):
+    def __init__(self, width=8):
+        # Save module parameters and instantiate generator and checker
+        self.width = width
+        hg = HammingGenerator(self.width)
+        hc = HammingChecker(self.width, correct=True)
+        self.submodules += hg
+        self.submodules += hc
+
+        # Create the intermediate bus and inject a single-bit error on
+        # the bus.  Position of the error bit is controllable by the
+        # error_bit input.
+        data = Signal(width)
+        error_bit = Signal(bits_for(width))
+        self.comb += data.eq(hg.data_in ^ (1 << error_bit))
+        self.comb += hc.code_in.eq(hg.code_out)
+        self.comb += hc.data_in.eq(data)
+
+        # Call out I/O necessary for testing the generator/checker
+        self.io = set()
+        self.io.add(hg.data_in)
+        self.io.add(hc.enable)
+        self.io.add(error_bit)
+        self.io.add(hc.code_out)
+        self.io.add(hc.data_out)
+
+gc = gen_check()
+print(verilog.convert(gc, gc.io, name="gen_check"))