Consider hiding label for methods with only one argument

instead of `Invoice.fromStr(s: "string goes here")` consider changing the method signature to

`class func fromStr(_ s: String)`

so the call becomes `Invoice.fromStr("string goes here")` instead. Thoughts?

Locate the Invoice.fromStr definition and its callers, then read how the current argument label is used. Decide whether the unlabeled signature is appropriate for this API, update the affected call sites and tests if the change is accepted, and verify the project still passes its test suite.

Written by the indexing model from the issue text.