bpo-37348 : add _PyUnicode_FROM_ASCII

[methane]

It is a convenient wrapper for _PyUnicode_FromASCII().
It can be used like PyUnicode_FromString, without a length of the string.

https://bugs.python.org/issue37348

[skrah]

I wonder if we need a new macro throughout the code base. In the case of _decimal, the paths are not performance sensitive, so I'd appreciate if you take it out.

If this really saves a measurable amount of time, we should have an external PyUnicode_FromASCIIString(), so other people can use it, too.

[methane]

In the case of _decimal, the paths are not performance sensitive, so I'd appreciate if you take it out.

OK, I reverted it.