A couple of preparatory commits:

r82025: In _Py_dg_strtod, 'e' now represents the adjusted exponent rather than the base exponent;  that is, the input value is of the form +- m * 10**e with 0.1 <= m < 1.0.  It's easier to produce such an 'e' from the parsing stage if we care about detecting overflow and underflow.

r82031: Update the s2b function:  remove a premature optimization in order to make s2b more general and its correctness more easily verifiable; alter the way that the input string is parsed so that it doesn't depend on nd0 being in the range [0, nd].