[SR-4861] `SingleValueEncodingContainer` and `SingleValueDecodingContainer` should have `encode<T>(_:)` and `decode<T>(_:)`

[norio-nomura]

Previous ID	SR-4861
Radar	None
Original Reporter	@norio-nomura
Type	Bug
Status	Resolved
Resolution	Done

Environment

swift-DEVELOPMENT-SNAPSHOT-2017-05-10-a

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	@itaiferber
Priority	Medium

md5: be28d8313133a12035d523766af2199e

Issue Description:

I'm implementing `YAMLEncoder` on jpsim/Yams#43
YAML has representations of Data and Date.
Date: http://yaml.org/type/timestamp.html
Data: http://yaml.org/type/binary.html
So, `YAMLEncoder` checks type in `encode<T>(_:)` and `decode<T>(_:)` whether it is `Data` or `Date`:
https://github.com/jpsim/Yams/pull/43/files#diff-7f4ff11f6b9e9c9f0412bd59e10f918bR229
to representing them in `YAML` spec.

YAML allows simple value as root document.
e.g. YAML document with single date value:

1970-01-01T00:16:40Z
...

But, `SingleValueEncodingContainer` and `SingleValueDecodingContainer` do not have generic `encode` and `decode`, so `Data` and Date` are encoded/decoded by each own's `Codable` implementations:
e.g.Single `Date` value is encoded as `Double`:

-9.783062e+8
...

To getting expected result from encode, I need `encode<T>(_:)` and `decode<T>(_:)` to `SingleValueEncodingContainer` and `SingleValueDecodingContainer` to allowing `YAMLEncoder` checking types as same as another containers.

[itaiferber]

This was introduced in PR #9871

[norio-nomura]

@itaiferber Thanks! Now Yams can code Data and Date directly without container. jpsim/Yams#43