Typealias RequestError to avoid KituraContracts import

[djones6]

Description

In a similar vein to #1217, this adds a typealias RequestError to Kitura, to avoid a user having to explicitly import KituraContracts when defining codable routes.

Motivation and Context

While creating a minimal code example of use of codable routing, @helenmasters noted that we need to import KituraContracts because of the RequestError? in the declaration of signature for a codable handler. This allows us to skip that import.

There are other situations where it may still be necessary to import KituraContracts, such as when defining a custom Identifier type, however the import can be confined to the source file that defines the type.

How Has This Been Tested?

A simple example of codable routing now compiles without importing KituraContracts.

The Kitura tests passed locally.

Checklist:

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

[codecov-io]

Codecov Report

Merging #1267 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1267   +/-   ##
=======================================
  Coverage   88.15%   88.15%           
=======================================
  Files          37       37           
  Lines        2355     2355           
=======================================
  Hits         2076     2076           
  Misses        279      279

Flag	Coverage Δ
#Kitura	88.15% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
Sources/Kitura/CodableRouter.swift	81.31% <ø> (ø)	⬆️

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[ianpartridge]

#1217 included a test, is that possible here?

[ianpartridge]

Looks fine in principle, would like a test.

[ianpartridge]

Do we need a dummy class like https://github.com/IBM-Swift/Kitura/blob/a8144547ed284befec973a0154790f208eb234d1/Sources/Kitura/HTTPStatusCode.swift#L20 ?

[djones6]

As far as I can tell, this dummy class was only necessary because the file otherwise contains only a typealias. In this case I dropped the typealias into the existing CodableRouter.swift since I could argue it belongs there.

[ianpartridge]

OK as long as Jazzy is generating the typealias OK then we're fine :)

[djones6]

Tested locally, it generates fine.

[djones6]

Hmm, that's interesting, my test fails to compile on Swift 4.0.3. It appears I inadvertently used some Swift 4.1 feature.