[Patch] Update _M_replace_safe/_M_replace_aux

[Paolo Carlini]

Hi,

this is a different kind of clean-up, also quite interesting ;)

Basically, I noticed that _M_replace_safe is called only with
third and fourth arguments either random access iterators
(_M_replace) or plain pointers. Second, I noticed that the
callers work in terms of size_types and pointers, not iterators.
Third, I noticed that internally _M_replace_safe computes
exactly quantities of the former types: this implies that every
time it's called (*very often*) we go through stupid conversions
from size_types and pointers to iterators and back.

Thus the below, which avoids a lot of redundant arithmetic for
append, assign, replace, insert, etc...

Once more, also a bug fixed: the check in _M_replace_safe for
length errors was incorrect (compare it to that in _M_replace_aux)

Tested x86-linux. As usual, will wait 12 hours or so.

Paolo.

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