In doing so, this removes all uses of the Encoder's stream operator,
except for where it is currently still used in the generated IPC code.
So the stream operator currently discards any errors, which is the
existing behavior. A subsequent commit will propagate the errors.
Currently, the generated IPC decoders will default-construct the type to
be decoded, then pass that value by reference to the concrete decoder.
This, of course, requires that the type is default-constructible. This
was an issue for decoding Variants, which had to require the first type
in the Variant list is Empty, to ensure it is default constructible.
Further, this made it possible for values to become uninitialized in
user-defined decoders.
This patch makes the decoder interface such that the concrete decoders
themselves contruct the decoded type upon return from the decoder. To do
so, the default decoders in IPC::Decoder had to be moved to the IPC
namespace scope, as these decoders are now specializations instead of
overloaded methods (C++ requires specializations to be in a namespace
scope).
In order to avoid the base encode/decode methods from being used (and
failing a static assertion), we must be sure to declare/define the
custom type implementations as template specializations.
After this, LibIPC is no longer sensitive to include order.
