APFloat storage complications

APFloat is derived from C code using fixed width storage for
the matntissa. When converting to C++ I changed it to variable-
width storage for space efficiency and generality reasons.

Unfortunately this leads to a complication during float->float
conversions that I missed that isn't present when using fixed
width storage.

Dale - I think this solves the issue correctly whilst preserving
correct rounding. Does it fix the problem case you had?

Neil.

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