Address the case of g_variant_builder_end()

[mzabaluev]

The documentation says:

It is not permissible to use [the GVariantBuilder instance] in any way after this call
except for reference counting operations (in the case of a
heap-allocated GVariantBuilder or by reinitialising it with
g_variant_builder_init() (in the case of stack-allocated).

In the bindings, this may mean:

• The binding method or function for this operation would consume some value owning a GVariantBuilder as a by-value parameter (likely self). It's interesting to support the stack-allocated case as well, so the method may need to be generic over some trait covering both cases.
• Normal semantics of grust::refcount::Ref would not work safely, as we need the reference to be unique by the time this call is made, or somehow drop all references immediately afterwards.

Same applies to g_variant_dict_end().