Add getFd method to FdInputStream and FdOutputStream

These methods are useful if you're receiving a generic stream in
application code and would like to check some non-stream behavior about
the file (e.g. stat or isatty).  Of course, downcasting can fail and
code should not rely on the file descriptor being available.

handle empty structs in isCanonical()

The trick here is that you must specify the value as a separate constant with a defined type. Then, you can reference that constant where an AnyPointer is expected.

Eventually we should maybe support some sort of inline syntax that specifies a type explicitly...

Support RPC on Windows (MinGW)

A grab bag of small changes to support the MSVC port

This is required to allow the compiler to successfully instantiate a
JsonCodec::Handler<DynamicValue, Style::POINTER> using Handler's default
partial specialization for Style::POINTER. Such an instantiation is
currently only used in dead code that clang and gcc eliminate but MSVC
does not, which is why the problem was not caught until now.

Also implemented DynamicValue::Builder::as<DynamicValue>() to preserve
symmetry.

For now, I'm just guessing on appropriate values for minStack and
maxStack.

The implementation used by AsyncStreamFd was simply calling the general tryRead(), so why not?

I prefer #ifdefing out the whole file rather than adjusting every build system.

Memory leak is due to compiler bug. See:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33799
http://llvm.org/bugs/show_bug.cgi?id=12286

Fix typo in KJ_DLOG macro.

Fix RealmGateway to avoid double-transforms on loopback.

Previously, in the presence of promise capabilities, a save() request could cross over the same gateway twice (or more!). Some gateways (e.g. Sandstorm's) implement one-way transformations, so this could cause the request to fail when it shouldn't.