Modifications to util::optional for KNL compilation

The Intel compiler was unable to compile the
`util::optional` code, and also created some warnings
in the unit tests for optional
* rephrase `enable_if` template argument to use `::value`
  explicitly to work around Intel compiler bug.
* move helper types out of anonymous namespaces in the
  optional unit tests
    * fix warnings about unused member functions
* remove some white space in optional code