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.

- Reduce noise.
- Trace through async callbacks.

A compatibility layer is provided to ease migration.

Distinguishing between "local bugs" and "preconditions" was proving difficult in practice, because a precondition failure in one function may very well indicate a bug in a calling function, but the exception may be thrown through that function, thus when caught the classification is nonsensical. The distinction also was not as useful as imagined. So, I eliminated this distinction.

Thanks to Bryan Boreham <bjboreham@gmail.com> for much help getting this started.

This prevents the compiler from reporting warnings in these headers while compiling application code. Hopefully this will stem the never-ending stream of complaints from people who enable pedantic warnings.

Add __forceinline for MSVC

For portions currently copyright by Kenton (most of it), transfer copyright to Sandstorm Development Group, Inc. (Kenton's company).

The license change is practically meaningless, as MIT and BSD 2-clause are legally equivalent. However, the BSD 2-clause license is sometimes confused for its ugly siblings, BSD 3-clause and BSD 4-clause. The MIT license is more immediately recognizeable for what it is.

Rémy Blank and Jason Choy (the two non-trivial contributors) are on record as approving this change:

https://groups.google.com/d/msg/capnproto/xXDd2HUOCcc/gbe_COIuXKYJ

Don't ever auto-define NDEBUG.  Just decide KJ_DEBUG based on optimization mode if neither DEBUG nor NDEBUG is defined.

Warn when an optimized build doesn't set NDEBUG and then #define it ourselves -- unless the build explicitly sets DEBUG.

Implement mutex and once-init synchronization primitives.  Wrap pthreads for now, but plan on raw futexes later.

Mark every destructor "noexcept(false)" unless it is very clearly noexcept.  The change in C++11 making destructors default-noexcept was a mistake.

Replace most CAPNPROTO_ASSERTs with the new logging/error macros.  Also add a SYSCALL macro and use it.  Also add commentary.