Optional decimal support

[JohnAD]

Is your feature request related to a problem? Please describe. I've written a decimal library at https://github.com/JohnAD/decimal128 that I will be using this week to add NumberDecimal (field type 0x13) to my fork of your bson library. I wanted to mention the decimal library just in case you would also like to add decimal to your built-in bson library.

Describe the solution you'd like You can either cut&paste any useful code you want; or even better, simply add the decimal128 library as a dependency via the nimble file.