Log message:
math/mpdecimal: import mpdecimal-4.0.0

Worked on in wip by Stefan Krah, bsiegert, adam, and myself.

libmpdec is a complete C implementation of the General Decimal Arithmetic
Specification.  The specification, written by Mike Cowlishaw from IBM,
defines a general purpose arbitrary precision data type together with
rigorously specified functions and rounding behavior.  As described in the
scope section of the specification, libmpdec will - with minor restrictions -
also conform to the IEEE 754-2008 Standard for Floating-Point Arithmetic,
provided that the appropriate context parameters are set.

libmpdec++ is a complete C++ implementation of the General Decimal Arithmetic
Specification. libmpdec++ is mostly a header library around libmpdec.  The
library frees users from manual memory management and has an easy API with
inline operators.  libmpdec++ has a thread local context for inline operators
and other functions that use the implicit context.