[Value] Scalar returns boost::optional< T > instead of raw value.
[Numeric] Simplify NumberConverter - its CanExactConvert() now inherits from NumberRelation.
[Value] Implement Scalar class.
[Value] Replace NamedValueEntry with Scalar.
[Value] (refactoring) NamedValueFeed ctors use move idiom.
[Value] (refactoring) Rearrange class layout of NamedValueFeed and NamedValueRef.
[Value] Add ConstNamedValueRef::AsFloat().
[Value] Scalar may accept const std::string& and std::string&& .
[Value] Scalar is now StringConvertible.
[Document] Implement IniSection with Scalar.
[Document] (refactoring) Remove code about "string array value".