Add fuzz testing using american fuzzy lop

Existing tests: block, transaction, signature, cold outputs,
cold transaction.

Data for these is in tests/data/fuzz.

A convenience shell script is in contrib/fuzz_testing/fuzz.sh, eg:

contrib/fuzz_testing/fuzz.sh signature

The fuzzer will run indefinitely, ^C to stop.

Fuzzing is currently supported for GCC only. I can't get CLANG
to build Monero here as it dies on some system headers, so if
someone wants to make it work on both, that'd be great.
In particular, the __AFL_LOOP construct should be made to work
so that a given run can fuzz multiple inputs, as the C++ load
time is substantial.

tests/CMakeLists.txt

@@ -77,6 +77,7 @@ file(COPY
   DESTINATION data)
 
 add_subdirectory(core_tests)
+add_subdirectory(fuzz)
 add_subdirectory(crypto)
 add_subdirectory(functional_tests)
 add_subdirectory(performance_tests)
@@ -114,6 +115,7 @@ set(enabled_tests
     hash
     performance_tests
     core_proxy
+    fuzz
     unit_tests)
 
 add_custom_target(tests DEPENDS enabled_tests)