Add CVE numbers to security advisories. Source: http://www.openwall.com/lists/oss-security/2015/03/17/3

Hardcode /bin/cat output option for `capnp compile`.

Implement CapabilityServerSet which provides a way to unwrap loopback capabilities to get the underlying server object, and also implements weak references.

Update comparison blog post to note that proto3 removes unknown field retention, for some ridiculous reason.

zero warnings: use KJ_UNUSED for unused constexpr & local vars

Ignore stuff that needs ignoring.

Unfortunately, commit 10487060 missed a case of CPU amplification via struct lists with zero-sized elements.

See advisory: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2015-03-05-0-c++-addl-cpu-amplification.md

one function in arena.c++ and several snprintf_result local vars
in string.c++ get optimized away because they're only used in
KJ_DASSERT() invocations.

this patch thus identifies those unused declarations with KJ_UNUSED

Deep-clear target object when overwriting pointer. (Minor bug I noticed while working on other things.)

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-0-all-cpu-amplification.md

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-1-c++-integer-underflow.md

Details: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2014-03-02-0-c++-integer-overflow.md