Mark constructors explicit

[sufferiing]

Marks the constructors for Quantity explicit to prevent code like this:

// all good
auto example(Length x, Angle y) {
    return x / y;
}

// BAD!
// values are implicitly converted into units via the constructor! could be very confusing
example(1, 2)

// GOOD
example(1_in, 2_deg)

from compiling with out specified units