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.

Disable UrgentObserver test on macOS.

Fix various problems reported by `clang++ -Werror`