Move UDLs out of the global namespace

[falbrechtskirchinger]

Moves the UDLs into nlohmann::literals::json_literals. See https://en.cppreference.com/w/cpp/chrono/operator%22%22h#Notes.

The UDLs can be brought into scope using any of the following lines:

using namespace nlohmann;
using namespace nlohmann::literals;
using namespace nlohmann::json_literals;
using namespace nlohmann::literals::json_literals;

Alternatively, the old behavior can be restored by defining the macro JSON_USE_GLOBAL_UDLS to 1, which puts the UDLs back into the global namespace.

Depends on #3590.
Fixes #1682.

To Do:

• Update documentation.
• Update uses of UDLs (examples, ...).
• Add unit tests for all using namespace lines.

[coveralls]

Coverage remained the same at 100.0% when pulling c94ed06 on falbrechtskirchinger:inline-ns-udls into d3e347b on nlohmann:develop.

[nlohmann]

Looks good to me.