Support stateful allocators, add `get_allocator` observer

[frederick-vs-ja]

Also

• add inclusion guard, making the header more usable;
• move alloc_guard_ out of basic_json, potentially making less specializations instantiated;
• rename destroy to clear as it behave like container's clear functions;
• destruct dynamically allocated strings, arrays, maps with allocator_traits::destroy.

See also [container.alloc.reqmts] and [allocator.requirements].

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Also replace runtime_error with json_error and add the json_errc enumeration type. Fixes #14.

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Also clarify the intent of default constructor of slices. Fixes #15.

[frederick-vs-ja]

The new commits address newly opened issues. @YexuanXiao