Add support for stdlib decimal.Decimal

[erichiggins]

I'd like to request adding the decimal standard library to micropython-lib.
https://docs.python.org/3/library/decimal.html

It's particularly useful for parsing and maintaining precision when reading values from peripherals (in my case, analytical balances) and displaying them on a screen without worrying about the issues associated with floating point arithmetic.

[jonnor]

This would be nice to have indeed!