Support bulk write on packing containers.

[bacek]

(This sits on top of std::span packing PR#23)

Packer: bulk write via if constexpr requires store_.write()

Optimise hot emit sites (emplace_integral, emplace_combined,
pack_type(string), pack_type(vector<B>, pack_type<span<B>>)) to call store_.write(ptr, len)
in one shot instead of iterating byte-by-byte through std::copy.

Falls back to std::copy for iterators without write(), so the change is
fully backward-compatible.

Add a general CTAD deduction guide for any output_iterator that exposes
a byte value_type, complementing the existing back_insert_iterator guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (aac15b8) to head (5d86b76).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   93.85%   93.91%   +0.06%     
==========================================
  Files           1        1              
  Lines         488      493       +5     
==========================================
+ Hits          458      463       +5     
  Misses         30       30              

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[rwindegger]

@bacek could you please add a unit test that covers the store_.write calls. Currently they are not called.

[bacek]

@bacek could you please add a unit test that covers the store_.write calls. Currently they are not called.

Fair call. Added.

[rwindegger]

Thanks for adding the unit tests. One last request please rename the test suite to match the other tests. It should be called msgpack23_bulk_write.

[bacek]

Thanks for adding the unit tests. One last request please rename the test suite to match the other tests. It should be called msgpack23_bulk_write.

Done.