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

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

Fixes #175.

Also set the version for the CMake build

Otherwise the installed pkgconfig files are broken. The release script
has also been changed to update the version for CMakeLists.txt.

Fixes issue #162

Allow DynamicStruct::Reader/Builder.as<AnyStruct>

fix 'capnp id' version string