Release 0.178.0

Defect fixes:

• Fixed issue with jmespath::join function through PR #546

• Fixed issue with the path for cmake config files through PR #547

• Related to #539, made the basic_json constructor basic_json(const Allocator&)
  consistent with basic_json(json_object_arg_t, const Allocator& alloc = Allocator()).

• Related to #539, basic_json copy construction now applies allocator traits select_on_container_copy_construction
  to the allocator obtained from other. For pmr allocators, this gives a default constructed pmr allocator rather
  than a copy of the allocator in other.

Enhancements:

• Improved the implementation of basic_json swap. Previously in some cases it would allocate.

• Improved the implementation of basic_json copy assignment. Reduced allocations when assigning from array to array and object to
  object.

• Documented the rules for basic_json allocators here,
  and added numerous tests for conformance to the rules.

• Added missing basic_json constructor

basic_json(json_array_arg_t, 
    std::size_t count, const basic_json& value, semantic_tag tag = semantic_tag::none, 
    const Allocator& alloc = Allocator());