Use `{}` rather than `()` within templates to avoid ambiguities

Reason

() is vulnerable to grammar ambiguities.

Example

template<typename T, typename U>
void f(T t, U u)
{
    T v1(T(u));    // mistake: oops, v1 is a function, not a variable
    T v2{u};       // clear:   obviously a variable
    auto x = T(u); // unclear: construction or cast?
}

f(1, "asdf"); // bad: cast from const char* to int

Enforcement

• flag () initializers
• flag function-style casts