fixed noexcept; added constexpr

add support for afl-fuzz testing

"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

replaced ssize_t occurrences with auto (fixes #204)

Fixed issue #199 - Small bugs in json.hpp (get_number) and unit.cpp (non-standard integer type test)