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

fixes #304

style-guide: fix isConst and UnConst examples

Problems:
* missing ; after struct declarations
* missing static for constexpr bool value
* UnConst_<T> -> UnConst_<const T>
* UnConst = -> UnConst = typename

json: integrate to build

In Sandstorm, we are encoding powerbox queries in packed base64 strings which may be placed in URL query parameters or the like. The strings are provided in interfaces called browser-side. We anticipate that some developers will prefer to specify a hardcoded string rather than generate it on-the-fly, since Cap'n Proto is not well-supported in browsers today, and anyway the developer may have no other reason to have a Cap'n Proto dependency at all, and powerbox queries are often static.

In this context, speed is irrelevant, while having a compact encoding is desirable. It felt sad to me to leave in the segment table in this context, adding redundant bytes when we want a compact encoding.

Fix uninitialized members of ListBuilder

Adding a `KJ_DASSERT` in the `setListPointer` logic flagged
non-word-multiple data sections in `INLINE_COMPOSITE` lists, which
should be impossible. This traced back to uninitialized member variables
in `ListBuilder` in the case that it was created from a null pointer.

kj/vector: add methods operator = nullptr and truncate

=nullptr releases memory

https://github.com/sandstorm-io/capnproto/issues/292