A wild idea that makes repeated string concatenations efficient without changing stringobject.c.

If we assume we don't want to change the representation of strings, then the problem is that string_concat(v,w) doesn't know if v will soon released, so it cannot resize it in-place even if the refcnt is 1.

But with some hacks ceval.c can know that.  Statements like s=s+expr or s+=expr both compile to a BINARY_ADD or INPLACE_ADD followed by a STORE_FAST or STORE_NAME.  So in the attached patch ceval.c special-cases addition of two strings (in the same way as it special-cases addition of two integers already).  If moreover the addition is followed by a STORE that is about to overwrite the addition's left argument, and if the refcnt is right, then the left argument can be resized in-place (plus some obscure magic to ensure that everything is still valid even if resize moves the string in memory).

With Python's good memory manager, repeated resizes even without manual over-allocation perform nicely.

As a side effect, other constructions like a+b+c+d+e+f also work in-place now.

The patch would do with a lot more comments.