New implementations of MochiKit.Format.truncToFixed and roundToFixed

[Per Cederberg]

Due to a new set of weird rounding errors in JavaScript floating-point
numbers, I opted to fully replace the MochiKit.Format.truncToFixed and
roundToFixed functions with new implementations. This basically
affects all of the other MochiKit.Format functions as well. The new
code pass all the tests in Firefox and IE, but "here be dragons"...
:-)

If you use this module in your code, please double-check that
everything still works as supposed in version 1.5 (r1491 or later).
Let me know if you bump into any issues.

Thanks,

/Per