Use absolute import path for c++.capnp

This allows {schema,rpc,rpc-twoparty}.capnp to be used without modification when
relocated outside of the Cap'n Proto codebase.

Add loadCompiledTypeAndDependencies() to SchemaParser so that compiled constants can be cast to static types.

Add ability to truncate a list post-allocation and reclaim space. Currently limited to blobs and can only be accessed through the Orphan interface.

I decided that consistent output across platforms is an important property, worth sacrificing a little bit of performance on MIPS.

Fix problem with the sockaddr struct.

Otherwise, at least clang++, forward-declares it inside the namespace `::capnp` and doesn't match it with a real `struct sockaddr` from the header 'socket.h'.

Also, accept `const` pointer - there is no point not to do so.

capnpc-capnp: keep list of extended interfaces.

Fix two typos in bit operations.

- Define separate times for durations vs. absolute times.
- Make more consistent with existing precedents in KJ style.

fix some error messages

Apparently writev() imposes a limit on the vector size.  Work around that.  Not sure why libc doesn't already do this.

Make it possible for a message builder to incorporate a byte array not owned by it without copying.  Useful for injecting large data blobs into a message without copying their contents.  Access this functionality through the Orphanage interface.