[Patch] Improve performance-wise __pad::_S_pad

[Paolo Carlini]

Hi!

To tell you the truth, I'm quite proud of this evening! ;)

In short:
  1- 4 less locals.
  2- Removed the __builtin_alloca.
  3- Avoid one traits::copy over two: now __oldlen chars are
     copied vs __newlen before, that is now we copy the bare
     minimum number of chars.
  4- Shorter and not less clean code, IMHO.

Indeed, the patch is quite invasive, and, whereas I have regression
tested it (x86-linux, as usual) and carefully checked it on paper,
please double check it! Thanks!

Paolo.

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