[issue27818] Speed up number format spec parsing

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I haven't found my old patch and tried to reimplement it. Added new file in stringlib. The original patch speeds up microbenchmarks only on about 4% on my computer (32-bit Linux). This is small, but the patch is simple. Moving some functions to template file adds yet about 2%. This is too small for adding new file. Thus I'll commit the original patch, with small style changes.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue27818>
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