Relax Codable Router type requirements

[djones6]

Description

Implementation of Kitura Evolution proposal: Relax Type Constraints for Codable Routing

Resolves Kitura/Kitura#1235 by relaxing the type requirements on the Codable Router to Decodable for input types, and Encodable for output types.

This enables a corresponding Pull Request in Kitura: Kitura/Kitura#1242

As this is merely a relaxing of type constraints, it can be delivered in a patch.

Motivation and Context

See Kitura/Kitura#1235.

The Codable Router was designed with a symmetry in mind for types to be shared between the client and server side (KituraKit <-> Kitura), which requires those types to be both Encodable and Decodable.

In addition, for types that contain purely Codable attributes, conformance to Codable can be synthesized by the compiler. However, there are cases where conformance must be implemented manually.

For cases where the user only wants to use type safety in a single direction (such as Decodable for a type that will only be received), we currently require them to implement conformance to Encodable (at least dummy conformance to satisfy the type requirement).

How Has This Been Tested?

Tests have been added to Kitura in Kitura/Kitura#1242 that are dependent on these changes, and those tests are passing.

Checklist:

• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.

[codecov-io]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           master   #18   +/-   ##
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  Coverage      82%   82%           
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  Files           6     6           
  Lines         489   489           
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  Hits          401   401           
  Misses         88    88

Flag	Coverage Δ
#KituraContracts	82% <ø> (ø)	⬆️

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[ianpartridge]

LGTM.

[djones6]

This is not quite ready. Although this is now correctly typed, there is currently no way for a user to POST a Decodable type, because the Codable Router currently requires that an Encodable type be returned in the response.

We need to have a think about how the API should permit this, which may require an additional closure type to be defined - such as allowing just an Id to be returned with no body, or allowing a (nil, nil) or (Identifier, nil, nil) response.

[djones6]

Kitura/Kitura@d8ebc69 relaxes the requirements for the Codable router result handler to permit a (nil, nil) response.

If this is deemed acceptable, then this PR is ready to go.

[djones6]

IBM-Swift/evolution#11 has been reviewed and approved.