Fuzzing module for various string operations, currently focusing on

encoding and decoding.

There are bunches of places where we don't want to invest in a full
fuzzer, but we would like to make sure that some string operation
can handle all its possible inputs.  This fuzzer uses the first byte
of its input to decide what to do with the rest of the input.  Right
now, all the possibilities are decoding a string, and seeing whether
it is decodeable.  If it is, we try to re-encode it and do the whole
thing again, to make sure we get the same result.

This turned up a lot of bugs in the key-value parser, and I think it
will help in other cases too.

Closes ticket 28808.

scripts/codegen/fuzzing_include_am.py

@@ -13,6 +13,7 @@ FUZZERS = """
 	iptsv2
 	microdesc
 	socks
+	strops
 	vrs
 """