json: implement decode(... input, DynamicStruct::Builder output)

# Conflicts:
#	CONTRIBUTORS

Fixed access violation in GetFunctorStartAddress()

Trivial changes required to support Visual Studio 2015 Update 2

Revert "Propagate DocComments into binary schema"

Propagate DocComments into binary schema

An immediately-invoked function expression with a default reference
capture list should be roughly equivalent to a GNU compound statement
expression. I'm unsure whether or not the result should be returned by
kj::mv() -- I would expect the compiler to optimize appropriately, but
have not tested this.

Note: There are two versions of KJ_REQUIRE_NONNULL in kj/debug.h: one that
works with MSVC's preprocessor, and one which works with the standard-
compliant preprocessor. This commit only touches the MSVC version of the
macro, because that is the minimum necessary work to get it to compile in
MSVC. However, it may make sense to convert the other one to use IIFE as
well.

This seems to work now in VS2015, and its presence causes assumption
failures (static assertions) in dynamic.c++.

json.capnp.{h,c++} must have been generated using an earlier version of
the code generator, or an earlier version of capnpc-c++.

Cloning capnproto/master, building and installing, then running
regenerate-bootstraps.sh results in this diff.

headers: check whether CAPNP_HEADER_WARNINGS is defined

Fix building with Xcode 7.3

Add observing for urgent data to UnixEventPort::FdObserver