box_value constructor: Replace param::hstring with hstring

[justanotheranonymoususer]

Partial fix for #1527.

Other places remain unfixed, example:

unbox_value_or(boxed, ReturnsStringView());

Or any other param::hstring usage.

[sylveon]

@DefaultRyan This PR is a good one - box_value will always do a copy of the hstring so we might as well skip the param::hstring here.

[DefaultRyan]

This breaks boxing of string-like types such as std::wstring and std::wstring_view via conversion to param::hstring.

I think we can still do this with a little more finesse. One idea of the top of my head is to have an exact hstring overload, and an overload for string-like types that are convertible to param::hstring. For the second overload, we don't actually create a param::hstring, but explicitly construct a fully-fledged hstring instead.