[issue14744] Use _PyUnicodeWriter API in str.format() internals

[Serhiy Storchaka]

Serhiy Storchaka <storchaka at gmail.com> added the comment:

> For Python 3.3, _PyUnicodeWriter API is faster than the Py_UCS4 buffer API and PyAccu API in quite all cases, with a speedup between 30% and 100%. But there are some cases where the _PyUnicodeWriter API is slower:

Perhaps most of these problems can be solved if instead of the boolean
flag (overallocate/no overallocate) to use the Py_ssize_t parameter that
indicates by how much should you overallocate (it is the length of the
suffix in the format).

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<http://bugs.python.org/issue14744>
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