Streamlined tuple "meta" algorithms

[trueqbit]

Originally, I wanted to create tuple "meta" algorithms in the internal "MPL" subframework:

• moved some tuple metafunctions to the mpl namespace (mpl::contains, mpl::counts, mpl::finds)
• updated tuple metafunctions for checking, counting and finding tuple elements (tuple_has, count_tuple, find_tuple)
• simplified mpl metafunctions and made them more robust
• [fixed] template-qualified calls to table_t::template for_each_column_excluding<>()
• improved implementation of column_constraints<Op...>::default_value()

Due to tests with early C++14 compilers, this PR also contains:

• qualified calls to std::get<>()
• use of the conservative trait value syntax is_trait<>::value again instead of variable templates, because of early C++14 compilers
• updated some places of the expression tests in a conservative way to remove the mention of compiler quirks (namely is_printable, is_bindable, is_field_of)

[fnc12]

Yay: pre New Year pull request from Klaus!