Fixes #1228. Fixes #234.

This uses imbue() to be locale independent.  Notes:

- 'sstream >> double' is much slower than strtod()
  * this was measurable in the test suite as a whole, despite being
    a tiny fraction of what the test suite does
- so, this embeds a fast path that bypasses sstream most of the time
  => the test suite is faster than before
- sstream is probably slower, because it does more accurate rounding than strtod()
- sstream does not create INFINITY by itself, this was done based on failure inferencing