Use std::enable_if as pfi::lang::enable_if

[ysk24ok]

This change contains backward incompatibility, so I've opened this as draft pullreq to get feedback.

Changes

This pullreq uses std::enable_if as pfi::lang_enable_if
because C++11 has std::enable_if and pficommon does not need to maintain its own implementation.

Backward Incompatibility

The template parameter of pfi::lang::enable_if and pfi::lang::disable_if has changed.

[ysk24ok]

In 924b5d4 , I added a document for pfi::lang::enable_if .
It's not deprecated so the usage and an example code are included in the document.

[takei-yuya]

LGTM!!