Remove ExpressibleByArgument conformance from Optional

[natecook1000]

Description

This drops the ExpressibleByArgument conformance for Optional. This conformance doesn't quite make sense, and allowed property declarations like this:

@Option(default: 0) var count: Int?

The default value meant that count would never be nil, and therefore shouldn't be optional.

Detailed Design

This change adds initializers specifically for when an Option's or Argument's value is optional, without a default parameter as well as deprecated versions that include a default parameter to guide clients to the correct declaration syntax.

Documentation Plan

The new initializers are documented; related initializers have been updated. There is no change required in the guides.

Test Plan

Existing tests exercise the functionality and verify the largely source-compatible nature of the change.

Source Impact

While this is technically source-breaking, it should have minimal impact in regular usage. All the declaration sites for arguments and options with optional values are still supported, with deprecation warnings for declarations that combine a default value and an optional type. If a property is declared as optional but does not have a default value, there won't even be a deprecation notice.

Code that uses Optional's conformance to ExpressibleByArgument, however, will break without warning. Users can work around this by adding an Optional-specific code path, or by other means.

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

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test

[natecook1000]

@swift-ci Please test