#101 use "static if" syntax for assertion requirements

[Lorenzo Caminiti]

Following N3329 "static if" proposal, replace assertion requirement syntax:

postcondition(
    back() == value, requires boost::has_eqaul<T>::value
)

with a static if:

postcondition(
    static if(boost::has_equal<T>::value) (
        back() == value
    ) // optional else ( ... )
)

[Lorenzo Caminiti]

Note that D already supports "static if" so it essentially already supports assertion requirements (even if maybe not actually documented for D's contracts).