Allow @Options with tuple types

[natecook1000]

This adds @Option initializers that use variadic generics to work with tuples of any size.

From the symbol documentation:

For example, you could use this property wrapper to capture the three dimensions for a package:

struct Package: ParsableArguments {
    @Option
    var dimensions: (Double, Double, Double)
}

A user would then specify the three dimensions after the --dimensions key:

$ package --dimensions 5 4 12

Still to do

• Decide on naming/etc on the help screen
• Add optional versions of the inits
• Include in the serialized help info
• Include in completion scripts
• Document in guides

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

[rauhul]

variadic generic code is damn near impossible to read...