[APFloat] Rename macro convolve => PackCategoriesIntoKey so that it is clear what APFloat is actually using said macro for.

In the context of APFloat, seeing a macro called convolve suggests that APFloat
is using said value in some sort of convolution somewhere in the source code.
This is misleading.

I also added a documentation comment to the macro.
[APFloat] Removed a assert from significandParts() which says that one can only access the significand of FiniteNonZero/NaN floats.

The method significandParts() is a helper method meant to ease access to
APFloat's significand by allowing the user to not need to be aware of whether or
not the APFloat is using memory allocated in the instance itself or in an
external array.

This assert says that one can only access the significand of FiniteNonZero/NaN
floats. This makes it cumbersome and more importantly dangerous when one wishes
to zero out the significand of a zero/infinity value since one will have to deal
with the aforementioned quandary related to how the memory in APFloat is
allocated.
[APFloat] Added make{Zero,Inf} methods and implemented get{Zero,Inf} on top of them.
[APFloat] Added support for parsing float strings which contain {inf,-inf,NaN,-NaN}.
[APFloat] Added a large unittest for APFloat.add that checks that special values are computed correctly.
[APFloat] Removed trailing whitespace from unittests.