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

- 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

Work around incompatibility between various C++ standard libs in C++14 mode on some Linux distros, including Ubuntu 14.04.

Also fix AnyStruct::Reader/Builder as methods

A compatibility layer is provided to ease migration.