Replace boost::{type_traits, enable_if and mpl} with standard library features...

This merge request replaces boost-type_traits, boost-enable_if and boost-mpl features with equivalent features from the standard library, as requested in issue #48 (closed).

However, in a few special cases, there is no reasonable alternative to using boost-type_traits and boost-mpl features, yet. These cases still use the above-mentioned boost features.