Improved function documentation and add new test

feat: add function to get currency code based on currency numeric code

AUD and SGD, HKD currency symbols

issue-156 update currency.go

issue-156 update constants.go

TZS must have 2 fraction digits according to ISO 4217

fix NewFromFloat to use newCurrency instead of GetCurrency

remove panic

Add variadic parameters

Add docs

Add Add,Subtract,Multiply many

Check ratios for integer overflow

Fix KGS currency template

GH-44 implement driver.Valuer and sql.Scanner for Amount and Currency (#99)

update workflow and run tests on latest go version