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

Cast mmap to char *.

Add abortRead to AyncIoStream

- Promise<T>::catch_() method which is equivalent to then() but you only specify an error handler; the non-error continuation is the identity function.
- kj::evalNow() function which synchronously calls a function and returns a promise for the result, or a broken promise if the function threw an exception. This is convenient for making sure exceptions get pushed into the promise chain.
- KJ_WARN_UNUSED_RESULT on various methods that return promises.

This is analagous to shutdownWrite, but for the read end of the stream