Fixes#108624
This allows `flat_map::insert(Iter, Iter)` to directly forward to
underlying containers' `insert(Iter, Iter)`, instead of inserting one
element at a time, when input models "product iterator". atm,
`flat_map::iterator` and `zip_view::iterator` are "product iterator"s.
This gives about almost 10x speed up in my benchmark with -03 (for both
before and after)
```cpp
Benchmark Time CPU Time Old Time New CPU Old CPU New
-----------------------------------------------------------------------------------------------------------------------------------------------
flat_map::insert_product_iterator_flat_map/32 -0.5028 -0.5320 149 74 149 70
flat_map::insert_product_iterator_flat_map/1024 -0.8617 -0.8618 3113 430 3112 430
flat_map::insert_product_iterator_flat_map/8192 -0.8877 -0.8877 26682 2995 26679 2995
flat_map::insert_product_iterator_flat_map/65536 -0.8769 -0.8769 226235 27844 226221 27841
flat_map::insert_product_iterator_zip/32 -0.5844 -0.5844 162 67 162 67
flat_map::insert_product_iterator_zip/1024 -0.8754 -0.8754 3427 427 3427 427
flat_map::insert_product_iterator_zip/8192 -0.8934 -0.8934 28134 3000 28132 3000
flat_map::insert_product_iterator_zip/65536 -0.8783 -0.8783 229783 27960 229767 27958
OVERALL_GEOMEAN -0.8319 -0.8332 0 0 0 0
```
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
34b2e934ea