Xsum library

The xsum library is imported from https://gitlab.com/radfordneal/xsum manually the directory contains a reduced version of the xsum library as required for the implementation of the Math.sumPrecise proposal since the library isn't expected to have frequent updates, the manual vendoring process was followed. In addition to importing select code a few modifications were made to the code:

• The files were updated from .c files (as found in the original repository)

to c++ files, .cpp.

• The code was updated to not use C specific syntax specifically the orginal

code extensively used the "restrict type qualifier" whose usage has been removed to comply with C++ syntax.