Use namespace std for int64_t and uint64_t

According to the c++11 standard, the declarations are within namespace
scope of the namespace `std'. Add `std::' to avoid unnecessary
requirement of implementations.

Corrected readme

Internally, the elements in a map are always sorted by its key following a specific strict weak ordering criterion indicated by its internal comparison object (of type Compare).
With string type, the default comparator is alphabetical order, and thus two come after three.

Create PULL_REQUEST_TEMPLATE.md

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)