godec: multi-precision decimal arithmetic for go

The API and the implementation are based on and complement those in the
multi-precision integer (Int) implementation in the Go library (math/big).

Limitations:
Support for formatting options is currently missing.
There is no support for potentially lossy conversions (e.g. float64 to Dec,
Dec to int64, or Dec to float64) as the requirements for these usually differ
depending on the use case.