Break ExpressibleByArgument default ambiguity

[natecook1000]

For types that conform to both RawRepresentable and LosslessStringConvertible, the provided default implementations of init(argument:) are ambiguous. This provides a doubly- constrained implementation that breaks that ambiguity.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary