Win32 build
Disambiguated vex::vector constructors

    vex::vector(size_t n)

should construct vector of size n with default
context.  This was overridden by

    template <class Expr> vex::vector(const Expr &expr)

in e5a378c4bce2f14163bb2f997ea8f3a19688a49d. Added SFINAE checks to
select the first version when appropriate.