Removing float arithmetic from amount.go

[dhwaneetbhatt]

Using float arithmetic with smaller numbers maybe okay because their precision is quite high and we use only 2 decimal points. But with larger numbers, the decimal precision is lost and we don't get back what we intend.

The following test before the code fix used to fail:

// very large number
amt, err = NewAmount("USD", "10000000000000000.20")
if err != nil {
	t.Error(err)
}
if v := amt.String(); v != "USD 10000000000000000.20" {
	t.Errorf("got %q", v)
}

amt.String() used to return 10000000000000000.00

[adamdecaf]

Thanks for this change! I cleaned up the code so our linters are happy.

[codecov-commenter]

Codecov Report

Merging #487 into master will increase coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #487      +/-   ##
==========================================
+ Coverage   55.73%   56.08%   +0.35%     
==========================================
  Files          71       71              
  Lines        2677     2687      +10     
==========================================
+ Hits         1492     1507      +15     
+ Misses        929      927       -2     
+ Partials      256      253       -3     

Impacted Files	Coverage Δ
pkg/model/amount.go	89.61% <100.00%> (+1.55%)	⬆️
pkg/upload/sftp.go	69.10% <0.00%> (+2.61%)	⬆️

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