Add Decimal support to ethers::utils::parse_units

[odyslam]

• Depends on Change wei from 1 to 0 #460 so that it parses wei accurately.
• It converts the number to float, performs the necessary computation, and finally, it returns U256 as it should
• It uses String as an intermediate step between f64 and U256 .
• I am unsure of the generic error that it returns. I added that because there are 2 types of Errors it could return (either from parsing string-->Float or from string-->U256).

If we agree with the general approach, I will change parse_ethers to be syntactic sugar for: parse_units with ether as the unit. This will add decimal support for that function as well.

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog