[Patch] Codecvt::do_length rewrite

[Paolo Carlini]

Hi,

this is the final part of the work, do_length... turned out to be
sligthly more difficult than expected, since mbsnrtowcs ignores
its fourth parameter if the first is NULL. Therefore, the best I
could do is adding a temporary dummy buffer. Interestingly, there
is *no* runtime penalty for this! Even more interesting and
unexpected, couldn't do a fare comparison with iconv, since,
_contrary to the official glibc docs_, passing a fourth NULL
parameter doesn't work and leads to seg faults or asserts. Weird.

Took to occasion to refine and uniformize here and there the existing
code for do_in and do_out. In the process I have discovered some
subtleties not completely covered by the current testsuite entries
and therefore mean to add a couple of new tests asap.

Tested x86-linux, will commit soon...

Paolo.

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