Clean up implementation of binary type

[nlohmann]

This PR polishes the integration of the binary type introduced in #1662 .

Changes:

• Added example to README.
• Extended examples for type checking functions like is_array, etc.
• Added a swap function.
• Created separate header file for byte_container_with_subtype. binary_t is an alias to this and replaces the previous internal_binary_t. There is no alias for the BinaryType template parameter any more - if needed, it is binary_t::container_type.
• Added conversions between JSON and binary type.
• Changed JSON serialization of binary types to object with bytes array and subtype integer/null value, e.g. {"bytes": [1,2,3], "subtype": 10}. Serialization to JSON is only for debugging purposes and should always produce valid JSON.
• Added test cases to achieve 100% coverage.
• Added get_binary function for simple access to the binary value by (const) reference.
• Rename json::binary_array to binary for symmetry to json::object and json::array constructors.

[coveralls]

Coverage increased (+0.06%) to 100.0% when pulling 4d39644 on binary_type into 5cfa8a5 on develop.

[OmnipotentEntity]

Looks good to me. Thanks so much for cleaning this up.

[nlohmann]

Looks good to me. Thanks so much for cleaning this up.

Thank you for implementing the feature in the first place!