Attached patch modifies text decoders to use the _PyUnicodeWriter API to factorize the code. It removes unicode_widen() and unicode_putchar() functions.

 * Don't overallocate by default  (except for "raw-unicode-escape" codec), enable overallocation on the first decode error (as done currently)
 * _PyUnicodeWriter_Prepare() only overallocates 25%, instead of 100%
for unicode_decode_call_errorhandler()
 * Use _PyUnicodeWriter_Prepare() + PyUnicode_WRITE() (two macros)
instead of unicode_putchar() (function)
 * _PyUnicodeWriter structures stores many useful fields, so we don't
have to pass multiple parameters to functions, only the writer

I wrote the patch to factorize the code, but it might be faster.