general announcement: adding decimal to Standard library

[JohnAD]

See nim-lang/RFCs#308

I'm planning on adding generic decimal type to the standard library and have general approval. As such, this project will very likely disappear (eventually) in deference to that one. I plan on still supporting the ideas of this library, including accurately supporting significance and scale/precision.

As a bonus, I'm going to attempt to support the IEEE spec natively as well: the variable will only use 128bits of RAM instead of the big variant type this one uses.