Try to stop the scanner of the maximum character before the end using a limit
depending on the kind (e.g. 256 for PyUnicode_2BYTE_KIND).

If we have at least one character in U+10000-U+10FFFF, we know that we must use
PyUnicode_4BYTE_KIND kind.

 * str[a:b] doesn't scan the string for the maximum character if the string
   is ascii only
 * PyUnicode_FromKindAndData() stops if we are sure that we cannot use a
   shorter character type. For example, _PyUnicode_FromUCS1() stops if we
   have at least one character in range U+0080-U+00FF

It is faster for long string and a little bit faster for short strings,
benchmark on Linux 32 bits, Intel Core i5 @ 3.33GHz:

./python -m timeit 'x=b"a"' 'x.decode("ascii")'
./python -m timeit 'x=b"x"*80' 'x.decode("ascii")'
./python -m timeit 'x=b"abc"*4096' 'x.decode("ascii")'

length |   before   | after
-------+------------+-----------
     1 | 0.234 usec | 0.229 usec
    80 | 0.381 usec | 0.357 usec
12,288 |  11.2 usec |  3.01 usec

(followup to Victor's 85d11cf67aa8 and 7a50e549bd11)

root, since the semantics varies across Unix variants.

root, since the semantics varies across Unix variants.

Use also _PyUnicode_READY_REPLACE() when it's applicable.

root (directory permissions are ignored).

root (directory permissions are ignored).

Reported and diagnosed by Thomas Kluyver.

Reported and diagnosed by Thomas Kluyver.

if the underlying raw stream is unseekable, even if the seek could be
satisfied using the internal buffer.  Patch by John OConnor.

if the underlying raw stream is unseekable, even if the seek could be
satisfied using the internal buffer.  Patch by John O'Connor.