Add value type support to as() and try_as()

[oldnewthing]

This permits fluent usage, which lets you read the code left-to-right instead of prefix usage which forces the eyes to dart back and forth.

Example of using as instead of unbox_value:

// old and busted with unbox_value()
auto h = unbox_value<Size>(SelectedItem().Tag()).Height;
auto h = unbox_value<Size>(propertySet.Lookup(L"key")).Height;

// new hotness with as()
auto h = SelectedItem().Tag().as<Size>().Height;
auto h = propertySet.Lookup(L"key").as<Size>().Height;

Example of using try_as instead of unbox_value_or:

// old and busted with unbox_value_or()
auto h = unbox_value_or<Size>(SelectedItem().Tag(), default_size).Height;
auto h = unbox_value_or<Size>(propertySet.TryLookup(L"key"), default_size).Height;

// new hotness with try_as() + std::optional
auto h = SelectedItem().Tag().try_as<Size>().value_or(default_size).Height;
auto h = propertySet.TryLookup(L"key").try_as<Size>().value_or(default_size).Height;

Including winrt/Windows.Foundation.h lights up these overloads. If you don't include it, then you get a compile-time error that points you to a line with the comment

// You must include <winrt/Windows.Foundation.h> to use this overload.

The new functionality is overloaded onto the existing names as and try_as to avoid introducing new names into com_ptr and IUnknown.

Factored unit tests to make it easier to add tests for the new overload. Also added a new unit test for com_ptr duck typing, which is an interesting corner case: You can use com_ptr for things that aren't actually COM objects, as long as they quack like a COM object. Care must be taken not to cast such objects to IUnknown, because the decoys won't have the same vtable layout (or possibly no vtable at all). Note such care in the new impl::try_as method, where we cast the raw pointer to com_ptr if it isn't a COM interface, thereby allowing the duck typing to proceed.

The trickiest parts are finding the right definition for the is_com_interface detector class and deploying it correctly via enable_if_t and if constexpr. The try_as overload which sets an output parameter in particular needs to detect is_com_interface + com_ptr. I reused the wrapped_type detector class for detecting com_ptr.

Also deleted apparently-unused is_detected detector class.

[oldnewthing]

Alternative to #688

[jonwis]

Are shouty-GUID and guid the same type?

[oldnewthing]

Sadly no. Shouty-GUID is the Win32 guid. Lowercase guid is the Windows Runtime Guid.

[kennykerr]

They couldn't be the same type because GUID is defined repeatedly as a macro in different places and that breaks C++ modules.

[jonwis]

What do you think about a defaulted version for default_value, so someone can say unbox_value_type_or_default<int32_t> directly, instead of passing in the default value?

[kennykerr]

All WinRT types have an "all zero bytes" representation for their default values, but I'm not sure about adding another function for this though. How common is it to want that default when that default isn't normally a usable value? And you can obviously always request that as the default.

[oldnewthing]

The tricky part is these enable_if_t's and if constexpr's. I'm still not confident that I got them all right.

[kennykerr]

Exhaustive testing of edge cases is the only way I feel confident about this stuff.

[kennykerr]

Looks great!