Reduce redundant code of prevector and speed it up
Reduce redundant code of prevector and speed it up

In prevector.h, the code which like item_ptr(size()) apears in the loop.
Both item_ptr() and size() judge whether values are held directly or
indirectly, but in most cases it is sufficient to make that judgement
once outside the loop.

This PR adds 2 private function fill() which has the loop to initialize
by specified value (or iterator of the other prevector's element),
but don't call item_ptr() in their loop.
Other functions(assign(), constructor, operator=(), insert())
that has similar loop, call fill() instead of original loop.

Also, resize() was changed like fill(), but it calls the default
constructor for that element each time.

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