Support risk policy parameter in constructors

[chiphogg]

Sometimes, end users have aliases for Quantity or QuantityPoint
types. In these cases, sticking exclusively to .as can be more
awkward than just using a constructor. Therefore, we add new Q-to-Q
(and QP-to-QP) constructors to support a second policy argument.

Follow-on for #387.

[geoffviola]

The changes look good. Thanks for cleaning that up.

sticking exclusively to .as can be more
awkward than just using a constructor.

What's the guidance there? Use fewer characters? 😁

[chiphogg]

What's the guidance there? Use fewer characters? 😁

Well, I won't complain about fewer characters! But I think of it more in terms of reducing complexity.

Example: we had one usage like this:

seconds_{value.template as<typename second_t::Rep>(seconds, ignore(TRUNCATION_RISK))},

where the type of the seconds_ member was the second_t alias. Of course, this is really equivalent to:

seconds_{value.template as<typename second_t::Rep>(typename second_t::Unit, ignore(TRUNCATION_RISK))},

So, we're kind of awkwardly recreating the target type second_t in a roundabout way. This both obscures intent, and creates opportunity for error and/or reader uncertainty.

This change allows us to write this instead:

seconds_{value, ignore(TRUNCATION_RISK)},

In fact, this example shows that this change is helpful for more than just alias use cases, because nothing about this final form depends on the fact that we used an alias to declare the seconds_ member variable.