Add support for afl-fuzz testing

[mykter]

I tested this library under afl last night: total of 400M executions, ~400 code paths found, no crashes or hangs. Stopped when it was >30mins that any of the fuzzers had found a new path. This is the code I used to support the fuzzing, in case you'd like to easily re-run against any future changes.
(I didn't test under ASAN, as cross compiling with -m32 didn't work out of the box)

"make fuzz" creates a simple executable that de-serialises stdin
and re-serialises to stdout.
"make fuzz_testcases" extracts the smaller json test cases into
a testcases directory.

The library can then be fuzzed as follows:
CC=afl-clang-fast make fuzz
make fuzz_testcases
mkdir out
afl-fuzz -i testcases -o out ./fuzz

[nlohmann]

Thanks a lot!