Move anticipated features from `phlex::experimental` to `phlex` namespace

[knoepfel]

This also includes moving the appropriate PHLEX_EXPERIMENTAL_* macros to PHLEX_*.

[knoepfel]

For prototype 0.1, the types that need to be moved to the phlex namespace include:

• phlex::[experimental::]configuration
• phlex::[experimental::]handle

The following macros should also be renamed:

• PHLEX_[EXPERIMENTAL_]REGISTER_ALGORITHMS