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

Apparently neither Morkdown nor Doctoc generates anchor tags consistently with Github itself; each fails in its own special ways.

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